feat(mail(web)): Templates folder for pre-defined drafts

To create a template, the user needs to:

- define a mailbox as the Templates folder;
- move a draft to this special folder.

Fixes #4320
Fixes #5363

UI/MailerUI/UIxMailFolderActions.m

@@ -832,6 +832,11 @@
   return [self _setFolderPurpose: @"Junk"];
 }
 
+- (WOResponse *) setAsTemplatesFolderAction
+{
+  return [self _setFolderPurpose: @"Templates"];
+}
+
 - (WOResponse *) expungeAction 
 {
   SOGoMailAccount *account;