Remove and ignore host-specific PNGs

These are site-dependent so they should not go in the repository
proper. Can replace with a simple image or script to generate
something reasonable-looking instead.
Saner (possibly; subjective) ordering for contests

More recent contests first. Ties broken by name.
Solve empty entry bug

Using an empty list as a fallback seems to do the job when we don't
want to include a given hashref in an array of other hashrefs.
Support indeterminate-length contests

Makes leaving an existing contest running forever less hacky because
it isn't necessary to set a ridiculously long duration any more.
Support 'open book' contests

In this kind of contest all past submissions by normal users will be
made available to view.

May want some kind of opt-out on this, or an intermediate mode where
users can only see solutions when they have solved the problem.
Save run.sh error logs to current directory

The old behaviour was confusing. Why log to ../?
CSS to support separate series/window themes
Change series display for openbook/indef contests

 - Openbook contests now have a class attached to their table row.
 - Indefinite contests don't show a progress bar any more.
 - The series view now has its own class.
Hide penalty field from indef contest scoreboard

Penalty can grow unreasonably large for indefinite-length contests.
Internally penalties are still used for ranking and this will continue
to be exposed to the template engine.
Link to other users' submissions if appropriate

This only needs to happen if the contest is open-book. For now,
anyway- post-contest expose still needs to be done by manually
changing the contest data.
Set site header based on current contest series

The default logo and redirect will be 'default.png' and 'bath14'
respectively. The bath14 part is a hardcoded hack which needs to
be changed- after adding some more UI it probably won't be necessary
for this to be a hyperlink any more.