Redesign: research only showing native scrollbars on hover

[bwindels]

Main problem so far has been the chunky scrollbars on windows that are always visible.
Edge actually has -ms-overflow-style: -ms-autohiding-scrollbar; to make them only visible on hover, for Chrome and FF we should be able to make it work with something like:

.scrollable {
  overflow: hidden;
}

.scrollable:hover {
  overflow: auto;
}

We should also detect whether this hack is needed so we don't do it on OSX for example.

Reasons for doing this:

• with gemini scrollbars we can't size scrollable areas relative to their content (like we want to do for room lists) and need to hack our way around this situation. Would be nicer to let css do the right thing.
• Lag because of how gemini scrollbars work
• Less complexity/code/smaller DOM \o/

[bwindels]

This would be replacing the gemini scrollbars just for the room sub lists, not across the app!