hgweb: allow defining Server response header for HTTP server
By default, the string contains the Python version. Server
operators may want to change this.
Because we like deterministic tests, we change the test harness
to always set this value to a known string. Various globs over
the server header have now been removed.
# no-check-commit because we add version_string()
Differential Revision: https://phab.mercurial-scm.org/D2840