Add batch actions and categories to admin UI for custom emojis (#11793)

app/controllers/admin/custom_emojis_controller.rb

app/javascript/styles/mastodon/tables.scss

app/models/custom_emoji.rb

app/models/custom_emoji_category.rb

app/models/custom_emoji_filter.rb

app/models/form/custom_emoji_batch.rb

app/views/admin/custom_emojis/_custom_emoji.html.haml

app/views/admin/custom_emojis/index.html.haml

config/locales/en.yml

config/routes.rb

spec/controllers/admin/custom_emojis_controller_spec.rb