IMAP source improvements

- Support multiple STORE commands. Defaults to +FLAGS (\Deleted)
  to conserve current behavior.
- Support custom search criteria. Defaults to NOT DELETED
  to converse current behavior.
- Support enabling/disabling IMAP expunge command
  after each message. Defaults to True to conserve
  current behavior.
- Increase functionality of the MockIMAPServer

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/sources/forms.py

@@ -120,7 +120,10 @@ class EmailSetupBaseForm(forms.ModelForm):
 
 class IMAPEmailSetupForm(EmailSetupBaseForm):
     class Meta(EmailSetupBaseForm.Meta):
-        fields = EmailSetupBaseForm.Meta.fields + ('mailbox',)
+        fields = EmailSetupBaseForm.Meta.fields + (
+            'mailbox', 'search_criteria', 'store_commands',
+            'mailbox_destination', 'execute_expunge'
+        )
         model = IMAPEmail