Fix: HTML and CSS refactor

The HTML and CSS in this repo is getting too messy and as we're trying to finalize modules now was creating too much technical debt. This PR will clean and refactor code as appropriate and bring it closer in line with our developing 18F HTML/CSS best practices.

- Normalizes spacing in default layout. Moved head tag to include.
- Cleans subpage layout. Pulls alpha mark and usa bug into separate includes for easier maintenance.
- Adds class-based standard typography and begins to implement across site for great consistency and code reuse.
- Adds responsive breakpoints.
- Normalizes to `rem` and adds mixin that take scare of fallback `px`
- Adds basic responsive code to map
- Adds basic responsive code to site title
- Removes `em` and other units from `line-height` across site as is best practice
- Cleans vertical scroll and related tooltips

Further refactoring will be necessary, but the code is now at a place that this can be done module by module going forward. As a result of refactoring, a few of the modules look slightly worse than before, but these problems will be addressed when we return to those sections.

@nickbristow: ready to merge now.