To allow an institution to have multiple themes available to it, this
change introduces a check on an optional theme parameter
$theme->institution. Only themes where $theme->institution matches the name
of the institution given, or where the value is not set are returned.

This change allows for multiple themes to be made available to an
institution.

The following behaviours should be seen:
* admin/users/institutions.php now only shows themes which are sitewide, or
  intended for the specified institution;
* admin/site/options.php shows all themes on the system (not just sitewide
  themes); and
* view/blocks.php lists all themes for all institutions a user belongs to,
  plus sitewide themes. This changes the behaviour introduced in
  4e0132ac

 which restricted available view
  themes to the currently set themes of each institution. However, I feel
  that this change offers more possibilities as institutions will be able
  to have a series of themes available for views; in addition to their
  primary Mahara theme.

Signed-off-by: Andrew Robert Nicols <andrew.nicols@luns.net.uk>