Update user management documentation and add UI functionality

There is no straighforward way of adding users to InfluxDB from the UI or the CLI. I had to google forums on how to add a new owner-level user. Found this:

Adding a ‘owner user’ - via API, as it’s not yet implemented in Influx CLI nor Influx web

```
curl -H "Authorization: Token $YOUR_AUTH_TOKEN" -XPOST \
-d '{"id": "$YOUR_USER_ID"}' $YOUR_INFLUX_HOST/api/v2/orgs/$YOUR_ORG_ID/owners
```

This needs to be added to the documentation for easier access, at least until it gets implemented in the UI.

##### Relevant URLs
- https://docs.influxdata.com/influxdb/v2.1/users/create-user/