Bug #805780

When a user's institutions are loaded, fetch the institution logos.
When choosing which of the user's institutions determines their theme,
ensure the same institution determines the replacement logo.  Finally,
if the page theme to be displayed is determined by the user, use the
user's header logo in the header template.

Change-Id: I710d6ff727d90276fc5008b179c60700f4499ccc
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>