Add field for reporter's Github handle in feedback form

[scossu]

I noticed that when issues coming from the feedback form are reported on Github, there is no obvious way for the reporter to know about them and possibly follow up with comments. It also may not be possible to pull them into the conversation if who is working on the issue doesn't know the reporter.

I can add a new field to the feedback form to provide the Github handle of the reporter, if he/she wants to be notified of progress or involved in the conversation. We disabled the return address for security reasons, but I wonder if this would be safe enough to implement (it would be hand-processed and the handle would be added as a mention to the ticket).

@thisismattmiller what do you think?