Eugen Rochko 4ec1771165 Add ability to specify alternative text for media attachments ()
* Fix  - Add ability to specify alternative text for media attachments

- POST /api/v1/media accepts `description` straight away
- PUT /api/v1/media/:id to update `description` (only for unattached ones)
- Serialized as `name` of Document object in ActivityPub
- Uploads form adjusted for better performance and description input

* Add tests

* Change undo button blend mode to difference
..
account_serializer.rb Change IDs to strings rather than numbers in API JSON output ()
application_serializer.rb Make sure nil IDs to not get cast to empty string in REST API ()
context_serializer.rb Refactor JSON templates to be generated with ActiveModelSerializers instead of Rabl ()
credential_account_serializer.rb Add setting a always mark media as sensitive ()
custom_emoji_serializer.rb New API: GET /api/v1/custom_emojis to get a server's custom emojis ()
instance_serializer.rb Uploads for admin site settings ()
media_attachment_serializer.rb Add ability to specify alternative text for media attachments ()
notification_serializer.rb Change IDs to strings rather than numbers in API JSON output ()
preview_card_serializer.rb Refactor JSON templates to be generated with ActiveModelSerializers instead of Rabl ()
relationship_serializer.rb Change IDs to strings rather than numbers in API JSON output ()
report_serializer.rb Change IDs to strings rather than numbers in API JSON output ()
search_serializer.rb Refactor JSON templates to be generated with ActiveModelSerializers instead of Rabl ()
status_serializer.rb Make sure nil IDs to not get cast to empty string in REST API ()