Add ability to follow hashtags (#18809)

...ontrollers/api/v1/featured_tags/suggestions_controller.rb

app/controllers/api/v1/followed_tags_controller.rb

app/controllers/api/v1/tags_controller.rb

app/controllers/api/v1/trends/tags_controller.rb

app/lib/feed_manager.rb

app/models/tag.rb

app/models/tag_follow.rb

app/presenters/tag_relationships_presenter.rb

app/serializers/rest/tag_serializer.rb

app/services/fan_out_on_write_service.rb

app/workers/feed_insert_worker.rb

config/routes.rb

db/migrate/20220714171049_create_tag_follows.rb

db/schema.rb

spec/controllers/api/v1/followed_tags_controller_spec.rb

spec/controllers/api/v1/tags_controller_spec.rb

spec/fabricators/tag_follow_fabricator.rb

spec/models/tag_follow_spec.rb