In e-resource results list fall back to using identifier from `issn` namespace if no identifier in `eissn` namespace

Summary:

On an attempt to display an identifier from the `eissn` namespace in the e-resource search and sort results list, we should fall back on using an identifier from the `issn` namespace if `eissn` is not available

User story statement(s):

As a erm librarian
I want to see an ISSN if it exists
so that I know I'm viewing the correct journal

Scenarios:

Scenario:

• Given a title instance
  • with subtype == electronic
  • with no identifiers in `eissn` namespace
  • with an identifier in the `issn` namespace
• When I see that title instance in the search and sort results list
• Then I should see the identifier from the `issn` namespace in the column labelled "ISSN (Online)"

(think this just requires a change in src/components/views/EResources.js)

Dev Task Breakdown:
Logic here needs to be extended to find the identifier from the `issn` namespace if the one from `eissn` is undefined