jsonld context cleanup

Closes #369

See merge request pleroma/pleroma!434

Small jsonld refactor

See merge request pleroma/pleroma!433

qvitter notifications: add new "read notifications" API

See merge request pleroma/pleroma!431

Runtime configuration

See merge request pleroma/pleroma!430

Mastodon API: relationships without id param

See merge request pleroma/pleroma!432

This copies Mastodon API behaviour & fixes Mastalab app.

Remove Access-Control-Allow-Origin in pleroma.nginx

See merge request pleroma/pleroma!424

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.

Unify Mastodon Login with OAuth login.

See merge request pleroma/pleroma!429

Runtime configured router

See merge request pleroma/pleroma!426

Get rid of Pleroma.Config in favor of Application

See merge request pleroma/pleroma!428

Same API as the old Pleroma.Config

This removes duplication in the login code.

Discussed in !426 (comment 7232)

Use the same example domain and use it as variable

See merge request pleroma/pleroma!423

Runtime emojis: Fix wrong return when finmoji is disabled

See merge request pleroma/pleroma!427

Runtime configured emojis

See merge request pleroma/pleroma!425

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()`.

Test for null-content activities.

See merge request !422