Implement MSC3952: Intentional mentions

[clokep]

MSC3952 defines push rules which act on a list of MXIDs in the event body, instead of searching the entire event body for display name / local part.

This implements the above in Synapse behind an experimental configuration flag. It does not yet implement the backwards compatibility pieces of the MSC.

[clokep]

There's still a couple of unknowns on the MSC which might affect:

• The exact format of the field (including if the length is limited).
• Backwards compatibility.

I think since this is disabled by default and subject to change that this is likely ready for an initial review and we can go from there.

[clokep]

(Also -- I have other experimental push changes and I don't want to conflict. 👍 )

[DMRobertson]

Generally LGTM 👍

IIRC the ability to send @room notifications --- or rather {"m.notify": {"room": true}} notifications --- can be (is?) gated based the sender's power level. Is that still the case after the MSC? I ask because I couldn't see anything about power levels here. I'm guessing Synapse enforces that when processing the event_push_actions_staging table?

[clokep]

IIRC the ability to send @room notifications --- or rather {"m.notify": {"room": true}} notifications --- can be (is?) gated based the sender's power level. Is that still the case after the MSC? I ask because I couldn't see anything about power levels here. I'm guessing Synapse enforces that when processing the event_push_actions_staging table?

Yes, I failed to include the extra check in the push rule definition though -- it is handled by a separate condition, (sender_notification_permission). Will update the PR. (Note that MSC is correct here.)

[DMRobertson]

LGTM, I'm happy if CI is!