tags: don't allow tags with unbalanced parentheses.

Don't allow tags to have unbalanced parentheses, except for a few
emoticon tags as special exceptions to the rule.

config/initializers/core_extensions.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Danbooru
   module Extensions
     module String
@@ -55,6 +57,22 @@ module Danbooru
 
         text
       end
+
+      # @return [Boolean] True if the string contains only balanced parentheses; false if the string contains unbalanced parentheses.
+      def has_balanced_parens?(open = "(", close = ")")
+        parens = 0
+
+        chars.each do |char|
+          if char == open
+            parens += 1
+          elsif char == close
+            parens -= 1
+            return false if parens < 0
+          end
+        end
+
+        parens == 0
+      end
     end
   end
 end