Make green widget selection bg that turns blue

And introduce the unicode snowman.

Test Plan:
Typed things like [[snowman radio 2]] in and saw that it's highlighted
in green (and blue when moving over it!).

Auditors: eater, alex
Add some facsimile of widget editing support

TODO: Make grading work.

Auditors: eater, alex
Add grading support for widgets in question

Auditors: eater, alex
Support custom answer type with widgets

Auditors: eater, alex
Widgets inside math don't work, so don't try

Auditors: eater, alex
Simplify onchange answer area logic a bit

Auditors: eater, alex
Add a simple add-widget button

Auditors: eater, alex
Small widget-related fixes

Auditors: eater, alex
Merge branch 'widgets'