Add rubocop and erb-lints rules for output safetey

This way we make sure we won't add `html_safe` or `raw` calls in the
future.

I'm excluding `text_with_links_helpers` for this check, because in this
situation the use of `html_safe` is justified: we check the original
input is safe, and we're only adding link tags to raw URLs.

.erb-lint.yml

@@ -1,5 +1,7 @@
 ---
 linters:
+  ErbSafety:
+    enabled: true
   ExtraNewline:
     enabled: true
   FinalNewline:

.rubocop_basic.yml

@@ -160,6 +160,12 @@ Rails/Date:
 Rails/HttpPositionalArguments:
   Enabled: true
 
+Rails/OutputSafety:
+  Enabled: true
+  Severity: warning
+  Exclude:
+    - app/helpers/text_with_links_helper.rb
+
 Rails/PluralizationGrammar:
   Enabled: true