Add federation handling for new reaction permission

[nickvergessen]

• Permission check in middleware does not account for federation
• Your instance is 34 but the host is 33
  • On update of your instance your permissions expend to add reaction permission
  • If the host changes your permission still on 33, it does not know reaction permission and will make you set your permissiions without reaction
  • Options:
    • 🐌 check in middleware if the server knows the permission via capability (very bad performance wise)
    • Do math when getting permissions update and check capabilities there (still not good, but less often)
    • Add the list of known permissions (max_custom number would be enough) to the request, so the instance can judge from there if it contains all numbers or needs to do some healing
• Host updates from 33 to 34 after you
  • We should let instances know that the users permissions got modified, but doing that in the migration might not work:
    • Server could be "not reachable" so requests could fail, etc
    • Sending many requests during the update is also not good
    • Next join of the participant should be good enough (we just need to make sure to track and notice the changed permission and store it locally?)
• Host updates from 33 to 34 before you
  • Currently the migration would simply change your permissions to now include the reaction permission
  • But what if the moderator took that away from you already?
  • Same as above we can/should heal the permissions when joining the room, that is most likely the most important moment and good enough

Originally posted by @nickvergessen in #16835 (review)