Adds isolated visual tests for javascript plugins (decoupled from docs). This makes it
faster and easier to test/develop js functionality not represented in unit tests, and gives us
a playground for interactions, etc.

It also makes it so developing javascript is now decoupled form jekyll, which should make everything
faster and less painful.

This commit also reverts my filter commit https://github.com/twbs/bootstrap/commit/9900771aa7f1b3ddcee49aec84082104776ace70
which broke scrollspy for dropdowns.
Merge pull request #13755 from twbs/fat-visual-test

Adds isolated visual tests for javascript plugins (decoupled from docs)....