LMS/Accounts: Converts many class-based Sass archetype-focused @extends to use Sass placeholder syntax

The close button for the modal dialog was marked up as a paragraph,
containing a graphical text character, inside two div elements, with an
associated click event. This made the button unusable for keyboard
users.

Replaced <divs> with a link element (<a role="button"), adapted the CSS
accordingly, and updated the existing <a> modal close buttons.

Bourbon's box-shadow mixin has been deprecated, and they recommend to use the
official spec.

The official border-radius mixins were deprecated and removed in Bourbon 3.0.
https://github.com/thoughtbot/bourbon/pull/95

This was previously done for input but not textarea.

This reverts commit 2b5e8a49, reversing
changes made to db0549be.

Change form style for login and signup modals so it is consistant in every browser and we have no usability problems

renames some scss files for better orginization, sets fixed widths on main containers to avoid akward scaling issues untill media queries can be added for more responsive scaling