Sidebar follow-ups

[kgilles]

The sidebar implementation is complete-ish. During development we noticed just how much space it may take up. We want to revisit it to use more of the freely available screen space outside the feed container when possible.

Long story, short: We should follow how Reddit has made the sidebar work well without taking up too much screen space.

The sidebar is also right now quite flat design-wise. We want to see if we can spruce things up a bit. Also by taking inspiration from other platforms.