Incomplete Mastodon API endpoint implementation breaks 3rd-party app OAuth flow

[DataDrivenMD]

Elk, along with several other 3rd-party apps, call GET /api/v1/apps/verify_credentials when they first register themselves with a Mastodon-compatible server (i.e. the app, not the user). This endpoint is not implemented in Wildebeest. This breaks OAuth flow during initial app registration.

In Elk, the missing endpoint triggers the following error:

{"url":"/api/wildebeest.example.com/oauth?origin=https://elk.example.com?code=xxxxxx","statusCode":400,"statusMessage":"App not registered for server: wildebeest.example.com","message":"App not registered for server: wildebeest.example.com","stack":""}

[DataDrivenMD]

Adding a clarification that this issue (#253) is distinct from issue #35 -- the incompatibility with IceCubes App described in issue #35 stems from an incomplete GET /api/v2/instance implementation. In the current issue, here, the problem stems from a missing GET /api/v1/apps/verify_credentials endpoint.