Pull modal cover and date utils into their own files.

Also move overview code into overview.js.

@singingwolfboy I think this is ready for review now. I moved more code into overview.js....

Things to test:
1) Modal dialog/cover in various scenarios (setting release date for section, editing course info and handouts, editing modules on unit page, uploading files on asset page).
2) Expanding and collapsing sections and subsections on the overview page.
3) Creating new sections and subsections (and escape to cancel them) on overview page.