notification client: look for a non-stripped room member event for invites

[bnjbvr]

The full invite event (including event id) might be present in the timeline section of a sliding sync response, so in theory we could look for a non-stripped room member event, and match the push notification event id against the invite event id from the timeline portion.

Needs a bit more investigation and confirmation by trying it out with a real-world synapse server.

[bnjbvr]

Note: the timeline events aren't handled by user-defined event handlers, when it comes to invited rooms. Adding support for that would be sufficient to solve the issue here, since both the SS proxy and native SSS seem to return the full invite event as part of the timeline (or required_state), for an invited room.