unless requested by admin or moderator.

Refactoring.

This copies Mastodon API behaviour & fixes Mastalab app.

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

This removes duplication in the login code.

The changes are a bit heavy since the emojis were loaded into module
attributes from filesystem.

This introduces a GenServer using an ETS table to cache in memory the
emojis, and allows a runtime-reload with `Pleroma.Emoji.reload()`.

When we lowercase the search it will succesfully do a case insenstive
match. Now #Linux will match #linux and #LINUX whereas previously it
would only match the exact case.

Better be sure than sorry

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex: Output an error when render(status.json) gives a nil