[OMP] Add ISBN to the Dublin Core Indexing Plugin

[kaitlinnewson]

Describe the bug
The Dublin Core Indexing Plugin generates meta tags in the HTML source of monograph pages which contain metadata for indexing services. In OMP we are currently missing the ISBN (e.g. DC.Identifier.ISBN) and should include this when present in the publication format metadata.

To Reproduce
Steps to reproduce the behavior:

1. Ensure the Dublin Core Indexing Plugin is enabled
2. Create a published submission which has ISBN populated in the publication format metadata
3. "View source" on the monograph page
4. See various metadata in the <meta> tags, noting the missing ISBN in the identifiers.

What application are you using?
OMP, all versions