BURs: refactor implication wiki page validations.

Move the validation that the tags in an implication must have wiki pages
back into the TagImplication model. Use validation contexts to only run
the validation when the BUR is created, not when the BUR is approved.

app/models/bulk_update_request.rb

@@ -73,6 +73,7 @@ class BulkUpdateRequest < ApplicationRecord
     def approve!(approver)
       transaction do
         CurrentUser.scoped(approver) do
+          processor.validate!(:approval)
           processor.process!(approver)
           update!(status: "approved", approver: approver)
           forum_updater.update("The #{bulk_update_request_link} (forum ##{forum_post.id}) has been approved by @#{approver.name}.")
@@ -107,7 +108,7 @@ class BulkUpdateRequest < ApplicationRecord
   end
 
   def validate_script
-    if processor.invalid?
+    if processor.invalid?(:request)
       errors[:base] << processor.errors.full_messages.join("; ")
     end
   end