CSS refactor

[Carlos-Muniz]

We were probably the last ones to use the old edx theme from Paragon (which helped us achieve the Studio look faster), but it's now gone, leaving many boxes and buttons unstyled. Refactor the styling so that it no longer uses the included vendor CSS, instead relying as much as possible on Paragon.

Acceptance criteria

• No page relies on vendor/studio.scss, vendor/normalize.scss or vendor/overrides.scss, but the general page layout is preserved. (For instance, buttons should be where existing users expect them to be.)
• Styling of individual page elements need not be preserved, but elements should be converted to their closest Paragon equivalents.
• If no Paragon component exists to replace legacy code, a new one should be create locally with a mind to eventually contribute it as a new one (or an update to an existing one).
• src/vendor is completely removed

[Carlos-Muniz]

The guiding principle is:

• Make the .scss in vendor/ as simple as possible by using vanilla paragon.
• Make everything consistent, not perfect.

[arbrandes]

@brian-smith-tcril, I've listed some acceptance criteria for us to discuss. The idea is for you to tackle this following the merge of #70.