Add invite responsible to form

It is added so that user can enter email ids of other users, who they want to invite to become booth responsible.

app/views/booths/_form.html.haml

@@ -12,6 +12,8 @@
           hint: 'e.g. employee, comunity manager, etc'
         = f.input :website_url
         = responsibles_selector_input f
+        = f.input :invite_responsible,
+          hint: 'This field is to invite unregistered users using their email to become booth responsibles.'
         = image_tag f.object.picture.thumb.url if f.object.picture?
         = f.input :picture