Extend MastoAPI to provide attachment mimetypes

See merge request !938

MastoAPI notifications is_seen

See merge request !934

Add robots.txt

Closes #723

See merge request !929

Add default robots.txt that allows bots access to all paths.
Add mix task to generate robots.txt taht allows bots access to no paths.
Document custom emojis, MRF and static_dir
static_dir documentation includes docs for the robots.txt Mix task.

Add `is_seen` to MastoAPI notifications, extract rendering logic into separate NotificationView, add tests

Stop mangling filenames and enable dedupe by default

Closes #715

See merge request !927

Fix delete activities not federating

See merge request !933

MastodonAPI.Admin.AccountView → AdminAPI.AccountView

See merge request !930

Add "local" & "tags" to users admin API

See merge request !932

Set content types settings on mastofe endpoint.

See merge request !894

Unify unfollow, accept and reject follow requests using CommonAPI

See merge request !931

Add credo to CI

See merge request pleroma/pleroma!902

This makes the output to be more compact while not loosing readability
--format=flycheck // --format=json could also be used but they are
not very human readable

Not really sure about the order in which this is but it feels right
because credo is more about checking if a ~working code could need
some improvements.<M-1>

Add "roles" to users admin API

See merge request !928