Notification Panel: Include Encrypted rooms (part 1: 24 hour POC)

### Your use case

The notification panel in Element web currently only displays unencrypted ping/highlight messages (mentions and keywords). The panel is extremely useful to people and internally we had a discussion [here](https://matrix.to/#/!kxwQeJPhRigXSZrHqf:matrix.org/$yuFgU3b3Da6GVA795ktBSf-F_b5gCkFTjhUVJEq9Rb4?via=matrix.org&via=element.io&via=vector.modular.im)

In the future we want the notification panel to:
- Include all messages you were mentioned or keyword-ed in, regardless of encrypted or unencrypted
- Move the access point (it's wrong on so many levels today 😂 )

Adding encrypted rooms to this list may take some work so we'd like to experiment with ways we might be able to do this. This issue is _only_ for the first experiment we've discussed.

---
**Can we add encrypted rooms to the notifications panel?**
- Yes, but performance will take a hit unless we make some changes to spec

Our suggestion:
- Have the notifications panel only keep messages from the last 24 hours. This reduces the load and therefore shouldn't have too much of a negative impact on the product. (In the future we should aim to remove or lengthen this cap, but for the purposes of testing how we'd do this, it's a good compromise for now)

---
**Open questions**

Things we're thinking about:
- If we shorten the unencrypted rooms 'memory' in the panel to only 24 hours user's may get upset
- If we add the two lists together users may get confused

To be discussed:
- Could we have a drop down at the top of the panel (similar to the threads panel) that let's users choose between 'forever unencrypted' and '24 hour encrypted' while we're in this experimental phase?
- Or, could we have a labs flag and measure how many people turn it on and are willing to only see the last 24 hours in order to get all their rooms?
- Or, could we extend the 24 hours to longer (maybe 3 or 4 days) and what would the performance impact/change be?

Spec land:
- What changes would we have to make server-side to help the client provide this to users?