BURs: don't allow renames for tags with more than 200 posts.

Don't allow tags with more than 200 posts to be renamed. An alias must
be used instead.

app/logical/bulk_update_request_processor.rb

@@ -1,4 +1,6 @@
 class BulkUpdateRequestProcessor
+  MAXIMUM_RENAME_COUNT = 200
+
   include ActiveModel::Validations
 
   class Error < StandardError; end
@@ -87,6 +89,8 @@ class BulkUpdateRequestProcessor
           tag = Tag.find_by_name(args[0])
           if tag.nil?
             errors[:base] << "Can't rename #{args[0]} -> #{args[1]} (the '#{args[0]}' tag doesn't exist)"
+          elsif tag.post_count > MAXIMUM_RENAME_COUNT
+            errors[:base] << "Can't rename #{args[0]} -> #{args[1]} ('#{args[0]}' has more than #{MAXIMUM_RENAME_COUNT} posts, use an alias instead)"
           end
 
         when :mass_update, :nuke

test/unit/bulk_update_request_test.rb

@@ -265,6 +265,14 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
           assert_equal(["Can't rename aaa -> bbb (the 'aaa' tag doesn't exist)"], @bur.errors.full_messages)
         end
 
+        should "fail if the old tag has more than 200 posts" do
+          create(:tag, name: "aaa", post_count: 1000)
+          @bur = build(:bulk_update_request, script: "rename aaa -> bbb")
+
+          assert_equal(false, @bur.valid?)
+          assert_equal(["Can't rename aaa -> bbb ('aaa' has more than 200 posts, use an alias instead)"], @bur.errors.full_messages)
+        end
+
         context "when renaming a character tag with a *_(cosplay) tag" do
           should "move the *_(cosplay) tag as well" do
             @post = create(:post, tag_string: "toosaka_rin_(cosplay)")