Custom Forms: Allow Chrome/Firefox feature that lets the user drag and drop into file inputs (#20033)

* Restored functionality of dragging files into .custom-file inputs.

* Replaced padding with height on .custom-file-input

Tried redoing this in flexbox, but it falls apart really quickly. Any changes in height of the cells throws it all off since the rows become columns (and thus, content across columns cannot be equally sized). Rather than implement something with such glaring flaws, I'm nuking it outright.

* Close dropdown menu when focusing an outside element
* Update unit test to new markup

remove magic dropdown positioning for fixed bottom; folks should set this as needed with .dropup modifier

- Renames .pos-f-t to .fixed-top, adds .fixed-bottom and .sticky-top
- Updates utilities Sass to be split across more files (one for position, sizing, and spacing)

Remove .navbar-divider; it's undocumented and wasn't updated with the flexbox navbar updates, so it's kinda bunk right now

- had to rearrange vars to re-use heading line-height
- align values while we're at it

* Update ie10-viewport-bug-workaround.js year

* Update narrow-jumbotron copyright year to 2017

* Update carousel/index.html copyright year to 2017

* Update browsers-devices.md copyright year to 2017

* Update change-version.js copyright year to 2017