Fix #4999: Unexpected error: ActiveRecord::RecordNotUnique sometimes appears when uploading posts

Fix two issues that could lead to duplicate errors when creating posts:

* Fix the submit button on the upload form to disable itself on submit, to prevent
  accidental double submit errors.

* Fix a race condition when checking for MD5 duplicates. MD5 uniqueness is checked on both
  the Rails level, with a uniqueness validation, and on the database level, with a unique
  index on the md5 column. Creating a post could fail with an ActiveRecord::RecordNotUnique
  error if the uniqueness validation in Rails passed, but the uniqueness constraint in the
  database failed. In this case, we catch the RecordNotUnique error and convert it to a
  Rails validation error so we can treat it like a normal validation failure.

app/models/application_record.rb

@@ -172,6 +172,16 @@ class ApplicationRecord < ActiveRecord::Base
         end
       end
     end
+
+    # Save the record, but convert RecordNotUnique exceptions thrown by the database into
+    # Rails validation errors. This way duplicate records only return one type of error.
+    # This assumes the table only has one uniqueness constraint in the database.
+    def save_if_unique(column)
+      save
+    rescue ActiveRecord::RecordNotUnique => e
+      self.errors.add(column, :taken)
+      false
+    end
   end
 
   concerning :UserMethods do