Element-web: Redesign: room header

[bwindels]

• [x] new icons
• [x] make edits work
• [x] fix content editable layout issues
• [ ] support expanding the header to show long topics
• [ ] reskin

Would probably make sense to postpone expanding header this till after #7581 is done, to avoid double work.

[uhoreg]

"support expanding the header to show long topics" will fix https://github.com/vector-im/riot-web/issues/1836

[bwindels]

@nadonomy I'm wondering what the expanded topic should look like. If we just make the whole header higher, that might take up a lot of space, as the room header is now above the right panel.

In the design there seemed to be some experimentation with a sort of overlay that didn't affect the layout of the timeline; rightpanel, or maybe I misunderstood that.