Add a configuration/landing page for onsite class list

A simple form is shown if you don't enter any GET variables.  It then
sends you to the class list with GET variables controlled by the form.
It's pretty rudimentary.

If you manually type GET variables into the URL, the behavior is the same
as before.

Addresses #349.
Add a refresh limit

This shouldn't be necessary as the class list should be fast enough on its
own, but it's pretty easy to limit the speed of refresh.  An integer (min.
number of seconds between reloads) can be entered into the
'onsite_classlist_min_refresh' Tag.

Addresses #351.
Merge pull request #1312 from learning-unlimited/onsite-classlist-tweaks

Onsite class list tweaks