[Port dspace-7_x] Show restricted thumbnails to users with access rights

[artlowel]

References

• 7.6 version of Show restricted thumbnails to users with access rights #4249

Description

There was already a call to get a shortlivedtoken in the thumbnail component. The issue was that in prod mode, angular didn't detect that the image src had changed afterwards so it didn't rerender the image.

The solution was to use a BehaviorSubject for the src and the isLoading boolean

Instructions for Reviewers

• Create a bitstream in the ORIGINAL bundle of an item
• Configure an embargo for that bitstream, with a date in the future
• Run filter-media -f -i ${the item's handle} on the item containing the bitstream
• Go to the item page as an admin, and verify that you can see that bitstream's thumbnail
• Go to the item page as an anonymous user, and verify that you see a placeholder instead

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 ESLint validation using yarn lint
• My PR doesn't introduce circular dependencies (verified via yarn check-circ-deps)
• 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 libraries/dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.
• If my PR includes new features or configurations, I've provided basic technical documentation in the PR itself.
• If my PR fixes an issue ticket, I've linked them together.

[tdonohue]

👍 Thanks @artlowel ! Verified this is the same code as #4251. Tested via #4251