Fixed OAuth restrictions for :api routes. Made auth info dropped for :api routes if OAuth check was neither performed nor explicitly skipped.

Automatic checks of authentication / instance publicity. Definition of missing OAuth scopes in AdminAPIController. Refactoring.

This reverts merge request !1877

Admin API: `PATCH /api/pleroma/admin/users/:nickname/credentials`, `GET /api/pleroma/admin/users/:nickname/credentials`.

Every time someone tries to use it, it goes mad and tries to scrape the
entire fediverse for no visible reason, it's better to just remove it
than continue shipping it in it's current state.

idea acked by lain and feld on irc

Closes #1595 #1422

- [] for options
- <> for mandatory arguments
- foo ... when foo can be repeated

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Closes: #1621

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