Design tweaks mostly based on https://www.loomio.org/discussions/3097

**this belongs to m. bartlet, i've just tweaked some bits to get tests passing. [origininal here](https://github.com/loomio/loomio/pull/803)**

Dashboard:

Lose start new discussion button (discussion here)
Gentler highlight colour
Make highlight work on unread lines (with a wicked ! important)
Groups panel typography & button to bottom
Nudge live pie to line up with fake pie
Discussion page:

Better vertical spacing in header
User-contributed text to black
Gentler chrome
Context panel & right-context panel tweaks
Discussion and proposal headers gentler
Activity list tweaks, including softening lines, inline names, icons out-dented
Lose double-quotes around proposals & discussion titles in activity list