make the Reply-To field configurable

This introduces multimailhook.{replyTo,replyToCommit,replyToRefchange},
that can be used for multiple purpose:

- disabling the notification

- setting the Reply-To to a particular email address (e.g. send the list
  to <project>-commits@list.server.com, but set the Reply-To to
  <project>-devel@list.server.com)

- changing the default strategy (pusher for refchange, author for
  commit).

We use the same variable names for all these scenarios, with the magic
values "author", "pusher" and "none". It is safe because the email
address has to contain a "@" and avoids introducing too many variables
for the same purpose.
Improve some documentation related to the replyTo feature.
Set the default values for the reply_to options in Environment.

Overwrite them in ConfigEnvironment only if configured.
Always use value "reply_to" in the templates.

There is no need to distinguish between "reply_to_refchange" and
"reply_to_commit" in the templates.  Instead, always use value
"reply_to" and just set that value differently in the two contexts.

This also allows the interface of Change.set_reply_to() to be
simplified a bit.
Merge branch 'reply-to'

Make the email "Reply-To" field configurable.