Robustify custom select

[chadwhitacre]

I made a custom select widget under #4305. There's work yet to be done on it to make it behave like a proper select.

• scroll widget into view when activated at edge of viewport
• arrow keys
• type to find
• scroll in touch - // http://stackoverflow.com/a/10803068
• ensure responsivity
• cross-browserize
• check on a phone, cf. Fix on a phone #4432
• bounce physics at end of scroll?
• prevent text selection?
• fix off-by-one bug on large lists: Clean up UX some more #4411 (comment)