fix: createfolders setting is ignored if readonly is True

If 'readonly' is True, folders shouldn't be created (regardless of
'createfolders' option). With old behavior, instead folders were always created
when 'readonly' is True (even if 'createfolders' was also False), which is a
serious bug (offlineimap was creating folders in all read-only repositories).
'createfolders' should only play a role if 'readonly' is False, in which case
folders should only be created if 'createfolders' is True.

Submitted-by: Vladimir Nesov <robotact@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

Changelog.rst

@@ -8,6 +8,7 @@ ChangeLog
 WIP (add new stuff for the next release)
 ========================================
 
+* Don't create folders if readonly is enabled.
 * Learn to deal with readonly folders to properly detect this condition and act
   accordingly.  One example is Gmail's "Chats" folder that is read-only,
   but contains logs of the quick chats. (E. Ryabinkin)

offlineimap/repository/Base.py

@@ -134,7 +134,7 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
 
         It is disabled by either setting the whole repository
         'readonly' or by using the 'createfolders' setting."""  
-        return self._readonly or self.getconfboolean('createfolders', True)
+        return (not self._readonly) and self.getconfboolean('createfolders', Tr
 
     def makefolder(self, foldername):
         """Create a new folder"""