Allow other modules to alter the query when returning valid contact form emails

In a project, we have a need to add some custom conditions to contact emails besides the default, which returns all enabled contact emails for a form. In our case we have a multi-regional (in addition to multi-lingual) site and each region wants to handle their form emails differently.

I found an easy way to do this by introducing a simple event in contact_emails that allows other modules to alter the query that's used. I'm testing this locally now and will submit a patch for review after that.