* Fix #117 - 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

* Update Yarn to version 1.1.0

* remove hard-coding

* Remove web UI warnings about OStatus privacy

* yarn run manage:translations

Call to warden.authenticate! in resource_owner_from_credentials would
make the request redirect to sign-in path, which is a bad response for
apps. Now bad credentials just return nil, which leads to HTTP 401
from Doorkeeper. Also, accounts with enabled 2FA cannot be logged into
this way.

* i18n: Update Polish translation

* i18n: Update Polish translation

- Previously they wouldn't receive it unless they were author's
  followers
- Skip unpush from public/hashtag timelines if status wasn't
  public in the first place

- 500.html generated with admin-set default locale if set
- Error page `<title>` includes Mastodon site title
- 500 title changed to "This page is not
  correct" (ref: <https://www.youtube.com/watch?v=2VCAP_seh1A>)
- 500 content appended with "on our end" to make clear it's
  not user's fault

A new rake task emojis:generate downloads a full list of valid
unicode sequences from unicode.org and checks it against existing
Twemoji files, finally generating a map from each sequence to the
existing file (e.g. when there's multiple ways an emoji can be
expressed). The map is dumped into app/javascript/mastodon/emoji_map.json

That file is loaded by emojione_light.js (now a misnomer) which
decorates it further with shortcodes taken from emoji-mart's index.

We added horizontal layout to preview card for wide image. However, max height of the thumbnail is still limited to 120px and it makes nearly square images to too small for that layout.

This PR increases max height as well as max width.

* Thread toot notification mails by conversation

* Make codeclimate happy and avoid potential mis-threading

* In detail status view, display attachment uncropped if there's only one

* Make media spoiler the size of the media it hides, enable on static

630px = Drawer + 1 Column

* Fix numbers in accounts columns

* Support edge compact-header

* Add emoji autosuggest

Some credit goes to glitch-soc/mastodon#149

* Remove server-side shortcode->unicode conversion

* Insert shortcode when suggestion is custom emoji

* Remove remnant of server-side emojis

* Update style of autosuggestions

* Fix wrong emoji filenames generated in autosuggest item

* Do not lazy load emoji picker, as that no longer works

* Fix custom emoji autosuggest

* Fix multiple "Custom" categories getting added to emoji index, only add once