implement cache on gradesheet page to save on change only

Faster Gradesheet (on-demand feedback and deferred rendering)

add searchable dropdown to create submission page

Add searchable dropdown and datepicker to extensions page

The file opened during local_submit is controlled by the student. Make
sure it's not a symlink. Uses O_NOFOLLOW, so no part of the path may be
a symlink

if the request came in over http (e.g. local_submit), then
the generated autograde_done url will be http, and will be rejected
when tango uses it

This reverts commit ec6a95d6.

This reverts commit c94258c2.

;

Updated wiki links

Added: Materialize in Statistics page. Tables are now a lot more elegant.
Added: Sample size for each grouping in Statistics
Added: Graphs for each grouping in Statistics
Removed: Clunky navigation for groupings and replaced it with a scrolling mechanism

Initial example directory