Favor #public_send over #send using Rubocop's cop

Devs keep using `#send` although that method does not preserve
private/protected visibility. Watching after this turned out to be quite
time-consuming while doing code review.

Currently, the Style/Send cop doesn't enforce `#public_send` however
(that's what we want). It simply discourages the use of #send. See
https://github.com/rubocop-hq/rubocop/pull/2081#issuecomment-292251650
for details. So a new entry on the Code Conventions doc has been added
to overcome this limitation:
https://github.com/openfoodfoundation/openfoodnetwork/wiki/Code-Conventions#prefer-public_send-over-send

.rubocop.yml

@@ -33,6 +33,9 @@ Style/HashSyntax:
   Enabled: true
   EnforcedStyle: ruby19_no_mixed_keys
 
+Style/Send:
+  Enabled: true
+
 Layout/MultilineMethodCallIndentation:
   Enabled: true
   EnforcedStyle: indented