Check for "Re:" at start of subject in case insensitive manner

To avoid a subject ending up like:

    "Re: RE: RE: RE: original subject"

That I've seen a few times.

The fix is to lower case the subject and the strings to compare against
before calling startwith()
On retagprompt, append a comma

So we don't have to remember to type the extra comma before typing a new
tag.

Conflicts:
	alot/commands/search.py
in retagprompt, sort the tags

So the tags are in a predictable order.

Conflicts:
	alot/commands/search.py
retagprompt: skip empty tags

(and so avoid having a leading comma)