include signature consistently for investigation form along with incident creation

previously, if you have an IR and make a new Incident and click in the upper
right on Investigation (so you create an incident and an investigation at the
same time) your Investigation lacks a signature, which is wrong.

see also #23707
respect config MessageBoxIncludeSignatureOnComment
don't include signature in Incident creation

Incident is *internal*, so it might be good to not include signature.
we can add an option later if it's arguable for flexibility.
Simplify Signature Inclusion

Now that Incidents have been hardcoded not to include signatures
(0efa2a4) because they're intended for internal consumption, remove this
variable.  Investigations are a simple "do I have content?  If not,
include a signature" check, similar to how RT's normal Ticket signatures
work.
Merge branch '3.0/consistent-signature' into 2.9-trunk