Escaping `/` and `::` in endpoint URLs

I tested out Jetpack sites installed into subdomains. In the existing (old) console it is pretty badly broken, but it is mostly working correctly here.

### Test cases

#### v1 API; enter `make.wordpress.org/core` into site field


This does not work - it sends a request to `v1.1/sites/make.wordpress.org/core/posts/` which is a 404 of course.

#### v1 API; enter `make.wordpress.org%2Fcore` into site field

This works correctly.

#### v1 API; enter `make.wordpress.org/core` into URL directly


This does not work, and we shouldn't expect it to work. There's no reliable way to tell that this is a URL for the `/posts` endpoint.

#### v1 API; enter `make.wordpress.org%2Fcore` into URL directly

This works correctly.

#### v1 API; enter `make.wordpress.org::core` into site field or into URL directly

This is sent to the server correctly, but the v1 API does not support it.

#### v2 API; enter `make.wordpress.org/core` or `make.wordpress.org%2Fcore`

This is sent to the server in the same way as the v1 API, but the v2 API does not support it.

#### v2 API; enter `make.wordpress.org::core` into site field or into URL directly

This works correctly.

### Potential improvements

1. If `/` is entered into a `site` URL field, transform it into `%2F`
2. If `::` is entered for the v1 API, transform it into `%2F`
3. If `/` or `%2F` are entered for the v2 API, transform it into `::`

No files or tests are named. Reproduce the listed v1 and v2 site-field and direct-URL cases in the React API console, then trace where endpoint URLs are assembled. Done means slash and double-colon inputs are converted appropriately for each API version and the listed requests reach the intended endpoint.

Written by the indexing model from the issue text.