Allow different types of messages to require different permissions

Inline media messages - particularly user-uploaded images and video - are significantly different in impact to text messages. In the troll-activity described in matrix-org/matrix-spec#564 the troll was both posting images and racist/threatening/offensive text, but the images have a much higher emotional impact and were much less easy to ignore.

Since media messages have the possibility of being much more disruptive than text, it might make sense to allow them to have a different moderation level - to allow people to post text but not images, for example.

Start by reading this issue alongside the referenced Matrix specification discussion, matrix-org/matrix-spec#564. Define the protocol-level permission change needed for media versus text messages and document what behavior and interoperability criteria would show the work is complete.

Written by the indexing model from the issue text.