Add categories for custom emojis (#11196)

app/controllers/api/v1/custom_emojis_controller.rb

...odon/features/compose/components/emoji_picker_dropdown.js

app/javascript/mastodon/features/emoji/emoji.js

app/models/custom_emoji.rb

app/models/custom_emoji_category.rb

app/serializers/rest/custom_emoji_serializer.rb

db/migrate/20190627222225_create_custom_emoji_categories.rb

...igrate/20190627222826_add_category_id_to_custom_emojis.rb

db/schema.rb

lib/mastodon/emoji_cli.rb

spec/fabricators/custom_emoji_category_fabricator.rb

spec/models/custom_emoji_category_spec.rb

yarn.lock