Don't allow pages with reserved slugs. Fixes #15.

For now it just complains on STDOUT. In The Future, it might be a good
idea to complain in some more highly-visible place, but snap logging is
kind of wonky (can't log except in a monad implementing MonadSnap, and
SnapletInit doesn't do that. hslogger?)
Set text/html content type.
Get most of the stray tags.
Replace whitespace with %20 in URLs. Fixes #16.

itsa now should produce valid HTML5, though there might be other
wackiness I haven't thought of yet.
Merge branch 'master' into theme-work

Conflicts:
	templates/default-layout.hamlet
Add some padding around the tag boxes.