Updated BS3 form classes

Once more I'm looking to update our approach to forms in BS3. Overall the goals here are:

* simplicity (in code and presentation)
* flexibility (not tying ourselves too close to specific HTML)
* consistency (using the same patterns as elsewhere in the project)

To that end, I've made the following changes:

* **Use `.form-control` for styling form controls instead of list of input types.** Fewer lines of code, less specific (meaning easier overrides as `input[type="text"]` is more specific than `.input`), less intrusive, and more performant.
* **Adds new `.form-group` class** to space out groups of form controls when in vertical mode. (Horizontal forms will continue to use the predefined grid classes.)
* **Updates form validation classes to separate HTML and CSS.** While not directly applicable to individual inputs, the markup is very flexible given the classes we use.

Still needs refinement I think, but overall I like it.