Fix new commit notification mails with non-ASCII (#1814)
    
    This is an alternative to #3140. As @puiterwijk figured out,
    when a new commit notification mail would contain non-ASCII
    text - i.e. when the commit author's full name or the commit
    topic contain non-ASCII text - sending the mail would fail. In
    3.x at least, this resulted in the service that sends the mails
    silently getting stuck; we don't know how the service behaves
    when this happens in current git (it was changed from trollius
    to celery), but the underlying bug is still there.
    
    It fails because of unicode conversion issues in the new commit
    notification function. `send_email` expects the body text to be
    provided as a `unicode` instance, but `notify_new_commits` works
    with `str` instances throughout (including the commit author and
    commit topic values; these ultimately come in as UTF-8 encoded
    strings, from the `pagure.git.read_output` function). Ultimately
    it produces a UTF-8 encoded `str` which `send_email` then tries
    to encode again, and that crashes.
    
    This approach fixes the problem in a way that's as consistent
    as possible with the other functions in the file - the body text
    is produced by `%s`-style string formatting of a unicode, and we
    ensure the things that get put into it are also unicode type,
    for the ones which can contain non-ASCII text. I also added some
    comments about this area, noted the expected type for `text` in
    the `send_email` docstring, and added a test.
    
    Fixes https://pagure.io/pagure/issue/1814
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>