refactor Topic validation

introduce a couple of custom validators
fix minor discrepancies in tests
copy I18n error message keys to default location
clean up validation invocation
move some responsibilities out of validator into class

lib/validators/quality_title_validator.rb

@@ -0,0 +1,9 @@
+require 'text_sentinel'
+require 'text_cleaner'
+
+class QualityTitleValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    sentinel = TextSentinel.title_sentinel(value)
+    record.errors.add(attribute, :is_invalid) unless sentinel.valid?
+  end
+end