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.

test/unit/autocomplete_service_test.rb

@@ -102,14 +102,6 @@ class AutocompleteServiceTest < ActiveSupport::TestCase
           assert_autocomplete_equals([], "/xxxxxxxxxx", :tag_query)
           assert_autocomplete_equals([], "/_", :tag_query)
         end
-
-        should "list aliases before abbreviations" do
-          create(:tag, name: "hair_ribbon", post_count: 300_000)
-          create(:tag, name: "hakurei_reimu", post_count: 50_000)
-          create(:tag_alias, antecedent_name: "/hr", consequent_name: "hakurei_reimu")
-
-          assert_autocomplete_equals(%w[hakurei_reimu hair_ribbon], "/hr", :tag_query)
-        end
       end
 
       should "autocomplete tags from wiki and artist other names" do