'Reply To' option for CiviMail in headers

Add a new, optional field to the CiviMail UI called "Reply-To" which
imitates the familiar "Reply-To" available in most email clients; apply
special behavior so that it coexists with CiviMail reply-tracking.

1. In the "Site Preferences" or "Mailer Settings", provide a checkbox to
"Enable Custom Reply-To".

2. In step 3 of the CiviMail wizard ("Mailing Content"), add a
"Reply-To" field adjacent to the "From" and "Subject" fields, with these
behaviors:

2.a. If the custom-reply-to is disabled per #1, then the field is hidden
we don't want to force users to think about this unless it's been
specifically enabled.

2.b. If reply-tracking (Step 2) is enabled for this mailing, then the
field is hidden – or, perhaps, it shows some immutable marker like
"Auto-generated" or the pattern "r.xxx.xxx@mydomain.com".

2.c. Otherwise, the field is a SELECT, and it presents the same options
as the "From" dropdown.