Performs linting and other general cleanup.

Performs cleanup based on css-linting recommendations, such as:
- removing units from 0 values.
- unseating two or more id selectors.
Removes unused style

Removes `drop-down-container` style, which was unused.
Moves popups to the component style directory

Popups were in the base styles. This commit moves it to it’s own
component style file.
Moves terminology box to component file

Moves terminology box to a component file. Adds general link styles to
links in the box.
Creates site-wide link mixin

Several areas of the app share the same link behavior of highlighting a
link when the cursor rolls over the main container containing a link.
This commit moves those styles to a mixin that can be shared among many
disparate parts of the app.
Adds bottom border to no results container

The no results container needed a light grey bottom border to be
consistent with the appearance of the page when the terminology box is
showing and no map is showing.
Adds code comments to CSS mixins

Adds additional code comments to mixins to make the comments consistent
with each other in regard to the arguments passed into the mixins.
Merge pull request #633 from codeforamerica/css-refactor

Css refactor