Extend MastoAPI to provide attachment mimetypes

See merge request pleroma/pleroma!938

MastoAPI notifications is_seen

See merge request pleroma/pleroma!934

Add robots.txt

Closes #723

See merge request pleroma/pleroma!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 pleroma/pleroma!927

Fix delete activities not federating

See merge request pleroma/pleroma!933

MastodonAPI.Admin.AccountView → AdminAPI.AccountView

See merge request pleroma/pleroma!930

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

See merge request pleroma/pleroma!932

Set content types settings on mastofe endpoint.

See merge request pleroma/pleroma!894

Unify unfollow, accept and reject follow requests using CommonAPI

See merge request pleroma/pleroma!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