Add per-user and per-instance keyword filtering

[ivanminutillo]

"A content filter is any kind of algorithm that ingests messages that arrive at the server and determines whether to block or flag a message based on certain predetermined criteria. Content filtering can be text-based or media-based."

[mayel]

For per-instance filters we can use MRF, or something like it if we want them to also apply locally.

For per-user filters I guess we could do same as user/instance silencing, but applied to an individual post/object which matches the filter (this check would only run once when the object is created, meaning filters wouldn't be retroactive though), by applying a boundary to the post which is equivalent to the author denying you read permission.

We could test this approach with:

• [ ] https://github.com/bonfire-networks/bonfire-app/issues/1009