Focus mode for video conferences

#### Is your suggestion related to a problem? Please describe.
In video conferences the space for widgets is quite small, especially on lower desktop screen resolutions.
Widgets like jitsi offers a fullscreen button, but this hides the chat, which is required for some facilitating methods.

#### Describe the solution you'd like.
For rooms containing video conference widgets there could be a button offered, which switches the layout to a minimized one:
- Hidden sidebars (LeftPanel, RightPanel)
- Hidden header
- Hidden rarely used controls
- Vertical aligned chat (less unused white space)

##### Proposal: Minimized Layout

##### Normal Layout for comparison

#### Additional context
This was already implemented in our [Element fork](https://github.com/zirkuszelt/frontend) in a quite hacky way. But hope to have something like this at some point upstream in an beautiful way :)
https://github.com/zirkuszelt/frontend/commit/6cc1ad3cd72504b65f5f7468c99d9a33ab2d86f7

#### Related issues
- #8851 Maximize Jitsi widget to whole chat area
- #15741 Allow widgets to use maximum vertical space
- #15984 Jitsi conference calls are completely unusable on a small screen