Fix z-index issue with thumbnails in item list checkboxes

[artlowel]

References

• Fixes Adding Relationship to existing Entity sometimes requires multiple clicks #1874

Description

Adds a z-index to ds-selectable-list-item-control to ensure it gets rendered higher than the thumbnail

Instructions for Reviewers

Verify that you can select a checkbox or radio button in the relationship modal

Checklist

• My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• My PR passes TSLint validation using yarn run lint
• My PR doesn't introduce circular dependencies
• My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• If my PR includes new, third-party dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.

[tdonohue]

👍 Excellent catch @artlowel ! You were 100% correct. The initial issue I had was simply that the thumbnail was overlaying the checkbox and I must have kept clicking on the thumbnail. Everything now works perfectly and there's no performance issues at all.