Avoid flashes of glitching when viewing rooms

[nadonomy]

When viewing a room, it's common to momentarily get flashes of different loading states or backgrounds, similar to a flash of unstyled content on the web.

We should avoid this, smoothing over visual glitches by:

• If showing a spinner in the timeline, only show it after several hundred ms (5-800?) of loading, to avoid it just flashing momentarily
• Alpha'ing in & out the spinners over 100ms or so
• Alpha'ing in the timeline content over 100ms or so, instead of it instantly flashing in