Add cache_function_for

This is a convenience alias for calling cache_function with a timeout_seconds
parameter.  Unfortunately it is not possible to possible to pass parameters to
cache_function when used as a decorator; now you can just do
`@cache_function_for(60)` to cache for a minute.
Add student registration "big board"

When student registrations are coming in, it's fun to watch how many students
have registered.  Unfortunately, 10 admins refreshing the dashboard is not the
greatest at making the site fast for users.  This page should be quick to load,
and is cached for a bit under 2 minutes, so many admins loading it is not a
problem.

There's a slight performance issue in one of the queries; I believe it
shouldn't be a problem in production, and it will be hard to fix until we are
on django 1.5.  See the comment starting on line 128 of
program/modules/handlers/bigboardmodule.py.  Other than that, the page is
fairly quick to load, and everything is cached.
Fix list of top classes by Priority/1

Now it will actually only count Priority/1 instead of all StudentRegistrations.
Remove some extraneous comments
Adds graph to studentreg big board.
Remove graph points in studentreg big board
Add gauges for load to studentreg big board
Add missing proxy = True to bigboardmodule
Switch to protocol-relative URL to load highcharts-more.js
Add 'students registering now' to the studentreg big board
Merge branch 'main' into studentreg-big-board
Merge branch 'refs/heads/main' into studentreg-big-board

Conflicts:
	esp/esp/cache/__init__.py
	esp/esp/cache/argcache.py
Fix merge of main
Add @needs_admin back to studentreg big board
Merge pull request #1412 from learning-unlimited/studentreg-big-board

Add student registration "big board"