Epic: Support embedded content #85

This ticket is an "epic" encompassing work we'll probably break up into separate smaller tickets.

Users should be able to embed some content into their posts. For example:

• Video file attachments.
• Videos from sharing sites like YouTube or TikTok.
• Audio files.

For safety reasons, the markdown renderer doesn't allow users to embed arbitrary HTML, so currently they're limited to image and links and can't embed things inline.

There are several different levels of support I could imagine us adding:

1. Automatically embed attached video (& audio?) files. (simplest)
2. Offer some way to embed links to external video.
   • Probably: some new protobuf data for embeddable links.
   • Simplest would to always put them at the end of a post, but could allow some special syntax for placing them?
   • Detect these kinds of URLs pasted into the markdown editor and "fix it up" into the right location?

As we implement different parts of this functionality, we should create separate tickets for those and link them here. Also, please feel free to comment with other use cases you think should be considered. :)