External Feature Layers cannot be templatized

Solutions.js currently fails when attempting to templatize a publicly accessible Feature Layer from an external org.

Steps to reproduce:
1. As any user in `Org A`, create a new `Feature Layer`, add some features, and publicly share
2. Create a new `Web Map`, add the `Feature Layer` from `step 1`, and publicly share
3. As an `admin` user in Org B, attempt to templatize the webmap.
4. Observe Solutions.js fails to templatize the items.

An API error is served for the following request:

Request:
```
curl 'https://servicesqa.arcgis.com/T5cZDlfUaBpDnk6P/arcgis/rest/admin/services/1609859260652/FeatureServer?f=json' \
-H 'authority: servicesqa.arcgis.com' \
-H 'pragma: no-cache' \
-H 'cache-control: no-cache' \
-H 'user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36' \
-H 'content-type: application/x-www-form-urlencoded' \
-H 'accept: */*' \
-H 'origin: https://test-1609860527645-qa-pre-a-hub.hubqa.arcgis.com:4200' \
-H 'sec-fetch-site: same-site' \
-H 'sec-fetch-mode: cors' \
-H 'sec-fetch-dest: empty' \
-H 'referer: https://test-1609860527645-qa-pre-a-hub.hubqa.arcgis.com:4200/' \
-H 'accept-language: en-US,en;q=0.9' \
--data-raw 'f=json&token=uCdJ_aCIuflwIO-2pWKOYp_hPVveJGocCJMOqnR6Hm8m9IdEuYS7m4VMO9wcyUP0hxV6ni16-ApRCVwoXVuy6PVO7ldmc8F9ywWJ1SAq2YRq8M_pjLtLM5QghH1rGnFkM4F6zN0Dxcu4gQrtgSEK0BPFJ2LS9BBXYyXgH-_NUxLJp2ILAFQypHGz1XTGJSM0Bc1Qh4Pp-VyvktatlLAX5ob0ztVYo4Yv3WIiEuwyLdk.' \
--compressed
```

Response:
```
{"error":{"code":403,"message":"User does not have permissions to access this service","details":["User does not have permissions to access this service"]}}
```

This appears related to [this line in getFeatureServiceProperties](https://github.com/Esri/solution.js/blob/develop/packages/common/src/restHelpers.ts#L991) path on the URL.

Removing `/admin` from the URL results in a success response from the API.

Request:
```
curl 'https://servicesqa.arcgis.com/T5cZDlfUaBpDnk6P/arcgis/rest/services/1609859260652/FeatureServer?f=json' \
-H 'authority: servicesqa.arcgis.com' \
-H 'pragma: no-cache' \
-H 'cache-control: no-cache' \
-H 'user-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36' \
-H 'content-type: application/x-www-form-urlencoded' \
-H 'accept: */*' \
-H 'origin: https://test-1609860527645-qa-pre-a-hub.hubqa.arcgis.com:4200' \
-H 'sec-fetch-site: same-site' \
-H 'sec-fetch-mode: cors' \
-H 'sec-fetch-dest: empty' \
-H 'referer: https://test-1609860527645-qa-pre-a-hub.hubqa.arcgis.com:4200/' \
-H 'accept-language: en-US,en;q=0.9' \
--data-raw 'f=json&token=uCdJ_aCIuflwIO-2pWKOYp_hPVveJGocCJMOqnR6Hm8m9IdEuYS7m4VMO9wcyUP0hxV6ni16-ApRCVwoXVuy6PVO7ldmc8F9ywWJ1SAq2YRq8M_pjLtLM5QghH1rGnFkM4F6zN0Dxcu4gQrtgSEK0BPFJ2LS9BBXYyXgH-_NUxLJp2ILAFQypHGz1XTGJSM0Bc1Qh4Pp-VyvktatlLAX5ob0ztVYo4Yv3WIiEuwyLdk.' \
--compressed
```

Response:
```
{"currentVersion":10.81,"serviceItemId":"949534d4b5644fdcab609e5b5a4ed507","serviceDescription":"","hasVersionedData":false,"supportsDisconnectedEditing":false,"hasStaticData":false,"hasSharedDomains":false,"maxRecordCount":2000,"supportedQueryFormats":"JSON","supportsVCSProjection":false,"capabilities":"Create,Delete,Query,Update,Editing,Sync","description":"","copyrightText":"","spatialReference":{"wkid":102100,"latestWkid":3857},"initialExtent":{"xmin":-14356986.047099045,"ymin":3328129.7896938594,"xmax":-6843031.7380439974,"ymax":5871843.5552304741,"spatialReference":{"wkid":102100,"latestWkid":3857}},"fullExtent":{"xmin":-14356986.047099045,"ymin":3328129.7896938594,"xmax":-6843031.7380439974,"ymax":5871843.5552304741,"spatialReference":{"wkid":102100,"latestWkid":3857}},"allowGeometryUpdates":true,"units":"esriMeters","supportsAppend":true,"supportsSharedDomains":true,"supportsWebHooks":true,"layerOverridesEnabled":true,"size":16384,"syncEnabled":true,"syncCapabilities":{"supportsAsync":true,"supportsRegisteringExistingData":true,"supportsSyncDirectionControl":true,"supportsPerLayerSync":true,"supportsPerReplicaSync":true,"supportsSyncModelNone":true,"supportsRollbackOnFailure":true,"supportsAttachmentsSyncDirection":true,"supportsBiDirectionalSyncForServer":true},"supportsApplyEditsWithGlobalIds":true,"supportsReturnDeleteResults":true,"supportsLayerOverrides ":true,"supportsTilesAndBasicQueriesMode ":true,"editorTrackingInfo":{"enableEditorTracking":true,"enableOwnershipAccessControl":false,"allowOthersToQuery":true,"allowOthersToUpdate":true,"allowOthersToDelete":false,"allowAnonymousToQuery":true,"allowAnonymousToUpdate":true,"allowAnonymousToDelete":true},"xssPreventionInfo":{"xssPreventionEnabled":true,"xssPreventionRule":"InputOnly","xssInputRule":"rejectInvalid"},"layers":[{"id":0,"name":"Point layer","parentLayerId":-1,"defaultVisibility":true,"subLayerIds":null,"minScale":0,"maxScale":0,"geometryType":"esriGeometryPoint"}],"tables":[]}
```