Universal share block (closes #2409)

@dcadams - this refactors the share block into one universal html file with variables set to change it's content or display style.

Places to look for appropriate behavior:
- home of class still in session (should display with just link to "show share block")
- home of class before start day (and join that class)
- home of class already finished (both with and without certs)
- quiz after having submitted and gotten score (same as last share block - was original functionality)