Rather than copy the basewiki around everywhere, it should be configured to
underlay the main srcdir, and pages be rendered from there if not in the
srcdir. This would allow upgrades to add/edit pages in the basewiki.

Implementaion will be slightly tricky since currently ikiwiki is hardcoded
in many places to look in srcdir for pages. Also, there are possible
security attacks in the vein of providing a file ikiwiki would normally
skip in the srcdir, and tricking it to processing this file instead of the
one from the underlaydir. -- Fixed by scanning srcdir first, then
underlaydir, and refusing to add any files from underlaydir if they also
exist in the srcdir. However, see [[security]] for caveats.

[[todo/done]]
