API: Create a "v4" API namespace

Instead of deprecating and changing the current API (V3), all breaking change issues will be around a new API version v4.

Grape's default behavior is fallbacking to the previous version endpoint, so we shall be backwards compatible by default on the unchanged endpoints.

I suggest setting our version to v4 as default on lib/api/api.rb. Doing this, we can literally copy existing behavior of "breaking changed" v4 endpoints to v3.