Reduce bitstream authorization requests for item page

[tinsch]

Description

This PR reduces the number of calls to the REST API by executing certain authorization requests only conditionally if the normal download is not possible for the user. Before, the authorization request "canRequestACopy" was always sent for every bitstream, now this only happens if the "canDownload" request returns false. This improves the response time performance for items with a large number of bitstreams and a large pagesize (>=50), as can be seen in a graph below.

Instructions for Reviewers

Currently the response time is slow for items with a large number of bitstreams. The current workaround are small pageSizes for the item's bitstreams per default. However, from a UX standpoint it would be nice to be able to show a larger list of bitstreams to the users, especially in the case of uploaded research data.

How to test this PR:

• create an Item with >=50 bitstreams
• edit some bitstreams, so that they are embargoed, or the access is otherwise restricted to anonymous users
• set item.bitstream.pageSize to 50 (or more)
• check that the response time is slower without this PR on the item page, and faster with the PR changes
• check that the "Request a copy" functionality still works for embargoed and otherwise restricted items

Here is a comparison of the reponse time of an item page, varied in the page size:

Related Issues, former issues with item bitstream pagesize:

DSpace/DSpace#8648
#2065

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Co-funded by the European Union and Universitätsbibliothek TU Berlin (DSpaceQPO project)

[tinsch]

Please note: I am not a frontend dev, so I would be grateful for a review by a more experienced frontend developer :) especially concerning the revised tests