Correct return value for makeDraftTiddler
Fix behaviour of new tiddler message

Resolves some inconsistencies over the behaviour of the new tiddler
message under various circumstances.

“new journal here” when a journal for today already exists now brings
up the existing journal for editing, and adds the required tag.

I’d be very grateful for any testing of the behaviour here: try using
new tiddler, clone tiddler, new here, new journal here, and new journal
in various combinations (eg with the draft not existing, already
existing, open or closed etc), and let me know of any peculiarities.