responses which tag/highlight users should use m.mentions instead of displayname-style

[NeilHanlon]

We currently mention/highlight users by just putting their matrix ID in the body of the message. Matrix 1.7 introduced "intentional mentions" which utilize a dedicated content field m.mentions with a key user_ids containing the set of users being mentioned (see https://patrick.cloke.us/posts/2023/12/15/matrix-intentional-mentions-explained/)

[GregSutcliffe]

This seems reasonable, I'll dig into it. Might relate to fedora-infra/noggin#1499 since it consumes FASJSON for the user data, but it makes it a good test case for that too :)