hgweb: consistently add search form to all gitweb pages
Paper has it on all pages. Not sure why gitweb doesn't. I think it
should be everywhere because it is a useful feature.
Also, we weren't consistently adding the HTML in the same place. This
was OK since the element is absolutely positioned. But this bothered
me a bit, so I went ahead and fixed it.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.