Make the bulk edit error messages specific to the type of error

- Before, everything would just say "Enter a valid URL to redirect to".
  Now it says "The URL to redirect to must be on a whitelisted domain"
  for a non-whitelisted redirect, for example.
Move everything from BulkBase into BulkEditor and delete BulkBase

- Nothing but BulkEditor uses BulkBase anymore.
Reword the blank redirect URL error message

- This makes for more consistency across bulk add and bulk edit, and we
  no longer need to append "New URL" to the error message.
- This also fixes the bug where the bulk edit modal was displaying the
  error message before the user having submitted the form.
Merge pull request #421 from alphagov/bulk_edit_consistent_error_messages

Make the bulk edit error messages specific to the type of error