Move common scss mixins to common/static/sass, and tweak the video player to have more in common between the cms and the lms

fixed custom tab box-sizing glitch in firefox

make codemirror use visible tabs

added moderator label styles

Fix to update the max_grade cache on StudentModule

* this may not be optimal (compared to e.g. replacing tabs with spaces), but it's a lot better than mystery 'bad indentation' errors

Feature/bridger/course grading

Feature/cdodge/cms static content management

Added HTML and Problem collapsibles

Remove Fellow and Course Manager positions from the jobs page

shimmy around the upload_asset widget to at least not mess up the edit unit rendering. Now it'll always be at the bottom

Add discussion tag

per feedback, use reverse() when generating callback urls. Also generate thumbnails when ingesting static images. We'll need these handy for the CMS UI.

Put tests to check ordering of scores returned on progress page.

Lyla/cms templates

Make it so we don't require HTTP basic auth to talk to the queue server -- I'm merging my own PR, so please check me -- I just need to do this to get the sandbox testing happy for 6.00.

Fixes multiple choice styling

Updates to grading doc.