Merge branch '210_twitter_api_uploads_alt_text' into 'develop'

[#210] TwitterAPI: alt text support for uploaded images. Mastodon API uploads security fix.

See merge request pleroma/pleroma!496

diff --cc lib/pleroma/web/activity_pub/activity_pub.ex
Simple merge

diff --cc lib/pleroma/web/mastodon_api/mastodon_api_controller.ex
Simple merge

diff --cc lib/pleroma/web/twitter_api/twitter_api_controller.ex
Simple merge

diff --cc test/web/mastodon_api/mastodon_api_controller_test.exs
Simple merge

diff --cc test/web/twitter_api/twitter_api_controller_test.exs
Simple merge

diff --cc test/web/twitter_api/twitter_api_test.exs
Simple merge