Notifications panel for aggregations (Reactions & Edits)

[nadonomy]

The notification right panel should display notifications that relate to aggregations. This would likely include:

• Receiving reactions to your messages
• Someone else (mod/admin) editing your messages (if Don't allow moderators to edit anyones messages #9793 hasn't landed)

And less likely include low signal:noise events like other users +1'ing reactions you've sent.

We should ensure we have a shared understanding of how notifications work in the right panel already (afaik mentions only, not all messages/noisy rooms) and then figure out how to expose aggregations from there.

If we group aggregations (e.g. "50 people reacted 👍 to your message [View]") we could consider grouping missed messages in all messages/noisy rooms in a similar way.

After figuring out how to move forward, we likely want to write up the plan of action in riot-meta to then feed work into all platforms.