Fix #5013: BUR model doesn't validate tags.

Don't allow users to request aliases, implications, or renames for invalid tag names.

As a side effect, it's no longer possible to request shortcut aliases like
`/hr -> hakurei_reimu` (slash abbreviations still exist, but they can't
be overridden with aliases). Tests involving these types of aliases are
removed.

config/locales/en.yml

@@ -80,8 +80,20 @@ en:
       messages:
         record_invalid: "%{errors}"
       models:
+        tag:
+          attributes:
+            name:
+              format: "%{message}"
+        tag_alias:
+          attributes:
+            antecedent_name:
+              format: "%{message}"
+            consequent_name:
+              format: "%{message}"
         tag_implication:
           attributes:
             antecedent_name:
               taken: "Implication already exists"
               format: "%{message}"
+            consequent_name:
+              format: "%{message}"