Add GET /api/v1/sites/teams endpoint and make related adjustments

[zoldar]

Changes

This PR adds a Sites API endpoint for listing teams the user is a member in. The team identifier from that endpoint can be used when either creating sites via POST /api/v1/sites to create a site under a particular team or scope the list of sites to a particular team with GET /api/v1/sites?team_id=....

The team identifier input was removed from Team Settings to avoid confusing users who don't actively use the API, which is the only place where identifier is explicitly used so far.

The endpoint is under scope which is implicitly available to all API keys. The api_available field on team entry determines if the team has Stats API feature enabled. If it's disabled, it's not possible to create sites under it (or, for that matter, run other API queries against its sites).

The GET /api/v1/sites endpoint was adjusted to scope sites to a team exclusively, if an identifier is provided.

TODO

• make updates to API docs after Document Sites API guests resource docs#591 is in extended docs in Document Sites API guests resource docs#591

Tests

• Automated tests have been added