Build: Reorganize external directory

Put each external library into its own directory.
Move jquery.js to external.

Ref gh-1266
Test: Get all jQuery versions from Bower

Ref gh-1266
Build: Upgrade to grunt-bowercopy 1.1.0

Ref gh-1266
Bower: Change jquery dependency to >=1.6 (don't limit max version)

Fixes #10110
Closes gh-1266
Tests: Use closeEnough() to account for rounding differences

Selectmenu's test suite broke with b0e8380, which changed the padding
used by the menu widget. Selectmenu conditionally adds pixels to the
menu's width, and using closeEnough() accounts for that and the
rounding differences across browsers.

Closes gh-1275