Add hidden field email form to help prevent spam bots

[MrDys]

CODEBASE-297: Add a field into the form that is hidden from display and is empty (or has a fixed string). If the contents of that field are modified from its original state we know that a bot has filled out the form and not a user.

[MrDys]

Original reporter: jkeck

[MrDys]

jrochkind: bullk change, pushing past 3.1 in preperation for 3.0.

[mejackreed]

Is this an issue for people still? @jkeck

[jkeck]

@mejackreed we have done and still do this in SearchWorks (as well as a few other Blacklight apps). I'm not sure anybody else particularly wants it but it wouldn't be too difficult to add if they did.