Add header to /u page

Added a header to to the /u portion of the site. The header is h1
and simply says "Users", with no extra styling.
Add header to /p

Added an h1, centred header to the /p page. This change is in
response to https://github.com/dlthomas/snowdrift/issues/193.
Merge pull request #281 from R-Morgan/master

Add header to /u page