Require .btn-default for regular buttons

* Instead of , use  for standard, gray button
* Improves ability to customize buttons by encouraging folks to not override .btn and thus overriding all button modifier classes
* Updates docs to reflect the change
* Reorganize buttons CSS
Fixes #7796: change @list-group-active-text to @list-group-active-color
nuke the unused navbar that was leftover
Update forms

* Overhaul the form control sizing section to only show sizing via grid columns as parents, not as classes on inputs
* Restore the inline-form option
* Restore the bottom margin on form controls and make them block level instead of inline-block
* More misc docs cleanup for forms
indentation fix
Merge pull request #7809 from twitter/bs3_adding_btn_default

BS3: Require .btn-default for all standard buttons
Fixes #7816: revert negative margin on .navbar-nav
hide entire navbar in print views
Drop typeahead.js--instead try Twitter's typeahead plugin
Super massive docs overhaul

* Bring back the navbar up top
* Pull the docs content back into separate pages because a 6.6k line docs file is just crazy
* Recenter the page content and bring back affixed side nav (needs work)