Remove log_environ calls for those warnings from ensure_sanity, so they don't appear on every page
Move all the admin warnings into a single function
Display warnings on the home page after the redirect from a successful site upgrade

Change-Id: I63f732ab9c0047ed70919eb6948c4fbf5d759b4b
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>