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

**this belongs to @mhjb , i've just tweaked some bits to get tests passing. [original here](https://github.com/loomio/loomio/pull/803)**

- John found a bug with user markdown preference not saving appropriately when making comments. fixed this in discussions controller 
- also on pie graph removed borders from legend keys and enlarged them slightly

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Dashboard:
* 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