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

- adjust some verbiage
- change the example to correct some labels and range names

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.