Remove permission checks using User Class system.

These changes will be undone when the new user group permission system is
complete. #2.
Fix the Account class's getID().
Add function to get an account by ID.
New 'User Group' implementation of permissions. #2
Break admin page into multiple pages.
Add user group administration pages. #2

This also adds the code to put new accounts into the default user group.
Link to user group admin page on admin homepage.  #2
Fix error message when removing user from group.  #2
Add anonymous access setting when creating group. #2
Add unimplemented newsgroup access control page.  #2
Add anonymous access setting to group permissions. #2
Fix setAccessToNewsgroup.
Add method to get a user group by name.
Implement newsgroup/usergroup admin page. #2
Fix getAccessToGroup().
Fix mostPermissiveAccessOf().
Apply user group permissions to sidebar list. #2
Apply permission settings using user groups. #2

This re-adds what was removed in 0d5a550.
Delete user memberships upon account deletion. #2
Remove user group associations upon newsgroup delete. #2