Use multiple layouts at the same time

## Additional context
I realized this could be useful because of [this](https://github.com/vector-im/element-web/issues/4635#issuecomment-746586903). This is going to gain importance if the bubble layout gets merged.

## The problem

A considerable amount of users would prefer to use the chat-bubble layout for 1:1 chats and at the same time to use the "Slack" layout for group conversations.

This can be applied to other layouts too.

## Solution

There are multiple ways to do this from the user perspective:

+ Set the layout for 1:1 and groups chats separately
+ Set the layout by some rules, example:
```
number of users <= 2: bubble layout
number of users <= 10: modern layout
else: IRC layout
```

Which one of these to use is a question of giving the user the correct amount (not too little, not too much) of options.

It would also be great to be able to overwrite the layout on per room basis.