Use quoted RFC822 string when autocompleting address: Bug #714956

Autocomplete now uses a properly-quoted RFC822 string when an address
is selected from the drop-down list.  This resolves the most egregious
case of the composer not accepting an email address formatted
[Last, First <mailbox@host>], as autocomplete is heavily relied-upon
and  Geary is presenting an address that can't be used, which is
confusing.

It's still possible for the user to manually enter such an address w/o
quotes (either typing or pasting it in).  However, this seems like a
far less problematic use case.  The ticket remains open due to this,
however.

Finally, some comments have been added to clarify the difference
between RFC822.MailboxAddress.get_full_address() and
to_rfc822_string().  I've also done a code-review to check that both
methods are being used correctly elsewhere in the code.