Recaptcha support added when reporting screenshots

There's lots of spam recently. We need to do something against it.

Gemfile.lock

@@ -208,6 +208,8 @@ GEM
       ffi (>= 0.5.0)
     rdoc (4.2.2)
       json (~> 1.4)
+    recaptcha (3.3.0)
+      json
     ref (2.0.0)
     responders (2.1.2)
       railties (>= 4.2.0, < 5.1)
@@ -291,6 +293,7 @@ DEPENDENCIES
   pg_search
   rails (= 4.2.0)
   rails-erd
+  recaptcha
   sass-rails (~> 5.0)
   sdoc (~> 0.4.0)
   slim-rails