Add eslint-plugin-jsx-a11y (#1651)

* 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).

app/assets/javascripts/components/locales/es.jsx

@@ -73,7 +73,7 @@ const es = {
   "notifications.column_settings.mention": "Menciones:",
   "notifications.column_settings.reblog": "Retoots:",
   "emoji_button.label": "Insertar emoji",
-  "privacy.public.short": "Público", 
+  "privacy.public.short": "Público",
   "privacy.public.long": "Mostrar en la historia federada",
   "privacy.unlisted.short": "Sin federar",
   "privacy.unlisted.long": "No mostrar en la historia federada",

app/assets/javascripts/components/locales/ru.jsx

@@ -16,7 +16,7 @@ const ru = {
   "status.show_less": "Свернуть",
   "status.open": "Развернуть статус",
   "status.report": "Пожаловаться",
-  "status.load_more": "Показать еще", 
+  "status.load_more": "Показать еще",
   "video_player.toggle_sound": "Вкл./выкл. звук",
   "video_player.toggle_visible": "Показать/скрыть",
   "account.disclaimer": "Это пользователь с другого узла. Число может быть больше.",