* Remove deprecated features at React v15.5

- [x] React.PropTypes
- [x] react-addons-pure-render-mixin
- [x] react-addons-test-utils

* Uncommented out & Add browserify_rails options

* re-add react-addons-shallow

* Fix syntax error from resolve conflicts

* follow up 59a77923b3

This introduces a new per-user preference called
"Auto-play animated GIFs", which is enabled by default. When a
user disables this setting, gifs in toots become click-to-play.

Previews of animated gifs were changed to display the video play
button so that users can distinguish them from regular images.

This setting also affects account avatars in the detailed account
view, which was changed to use the same hover-to-play mechanism
that is used for animated avatars in timelines.

Fixes #1652

* Add eslint-plugin-jsx-a11y.

* Fix npm script.

* Adjust npm scripts so test also runs lint.

* Fix existing lint errors.

* Don't break on a11y issues.

* Add role and tabIndex.

* Add vim and Mac files to .gitignore and .dockerignore.

* Handle htmlFor (partially), a that's actually a button.

* Fix missing tabIndex.

* Add cursor:pointer to load-more

* Revert change to load_more.

* Fixes based on review.

* Update yarn.lock.

* Don't try to install fsevents on Linux (hides warning noise).

Technically the check for iOS from components is a side effect, because it's
neither in state nor props, but the user agent does not change, so I think
that's okay.

before. In the API, attachments now can be either image, video or gifv. Gifv
is to be treated like images in terms of behaviour, but are videos by file
type.

adding attachments display to static views