Bugfix: Don't prepend user id to contact.email

Story: https://www.pivotaltracker.com/story/show/67887120

I've spent more than an hour on this, but couldn't figure out why the contact id gets prepended. I did notice that it only happens on the first time a mustache variable is inserted into an input field hence the solution with the hidden input field.

I've also discovered that this doesn't happen if you remove the `refresh_queue.enqueue(arg.reify());` call in the `#using` mustache helper ([here](https://github.com/reciprocity/ggrc-core/blob/develop/src/ggrc/assets/javascripts/mustache_helper.js#L1021)).

Let me know if I should spend more time on this or if somebody has any idea what's going on :confused: