Type-based feed filtering

[EricBAndrews]

This one comes from the Matrix community: add an option to display only certain types of content (images, links, text). Should be quite straightforward.

[Sjmarf]

Where do we put this setting? In the sort mode menu, or in the ellipsis menu? Or should we rethink our approach entirely? Perhaps we could shift some of the settings that are currently under the ellipsis menu (community-related options like subscribe, favorite, etc) to a dropdown attached to the navigation title, like we have for subscribed/local/all feeds? That makes the ellipsis menu only include view-related options, which is arguably a more user-friendly design.

At one point I had a sort-mode menu redesign PR (#589) that this would fit into nicely, but I haven't been keeping it up-to-date.