Corrected a couple of spelling mistakes in error language strings. Moved wwwroot URLs into the language strings as proper interpolated get_string parameters.
(cherry picked from commit eb80727a)

Hide institution authplugin dropdown for institutional admins; print warning when no auth plugin is configured

along with all the other settings

still to do:

- cron processing is completely untested
- i would like to split out the import table based on the transport:
  import_queue.host and token should go into a import_queue_mnet table
- it's possibly worth thinking about making import a proper plugin type.
  not sure about the effect this has on the import transport
  framework...  it might be possible to have both import and
  import transport plugintypes but that might be too heavy
- at the very least if we split out import_queue.host and token into an
  mnet table it will pave the way for a better refactor laterz.
- i would still really like to improve the mnet namespacing but that
  might be plausible at this point.
- need to write docs about arguments and return types
- i want to change the content_ready arguments to not include
  $filesmanifest as that is dependent on format being file - it may
  be actually better to dispatch somewhere else based on $format and
  then just have a generic $data which would be $filesmanifest for files
  and then something else for something like LEAP or maharanative or
  whatever, as this is checked in the importer, not not the
  importertransport.

Prevent role changing in groups if the user is the only admin. Add some functions to abstract role changing.

This patch adds:

 * group_is_only_admin($group, $user)
 * group_can_change_role($group, $user, $newrole)
 * group_change_role($group, $user, $newrole)

These functions are used on changerole.php, and also on the member listing page to prevent showing the change role link if the user is the only admin in the group.

If the user has no roles that they can change to and changerole.php is visited, it will redirect back to the member listing with an informational message.

Prevent users from leaving a group (in group_remove_user) when they are the only admin in the group.

Add caching to group_user_can_leave.