rich media: add helpers module, use instead of MastodonAPI module

lib/pleroma/web/mastodon_api/mastodon_api.ex

lib/pleroma/web/mastodon_api/mastodon_api_controller.ex

lib/pleroma/web/mastodon_api/views/status_view.ex

lib/pleroma/web/rich_media/helpers.ex

...roma/web/twitter_api/representers/activity_representer.ex

lib/pleroma/web/twitter_api/views/activity_view.ex