Use of embed sizes in the ComCol tree

[MarieVerdonck]

References

• Fixes more than 150 GET requests when navigating to Communities & Collections view #918
• See also Embedded lists don't have pagination options in their self link DSpace#3174

Description

The communities and collections expandable tree at {baseUrl}/community-list has been refactored so the requests required to find out if a node in the tree has children (i.e. if a community has sub-communities or collections) are now embedded in the initial request with the new embed and nested embed size functionalities. (see DSpace/DSpace#3114)
This should reduce the number of requests needed for rendering the ComCol tree nodes and their shevrons to the ones mentioned in the review instructions (fixes issue #918)

Might be an issue with the embed sizes resulting in non-identical result alternative links (see DSpace/DSpace#3174), but doesn't seem to be an issue in this use of the functionality.

Instructions for Reviewers

• Go the communities-collections tree at {baseUrl}/community-list and open network tab:
  => Should send only one request to /communities/search/top with embedded collections/subcommunities with embed.size 1.
  => No additional requests (with size=1 like before) should be needed
• Expanding any node should result in just 2 requests: the collections and the subcommunities, again with its collections/subcommunities embedded with size 1.
  => No additional requests (with size=1 like before) should be needed

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]

👍 @MarieVerdonck : Tried this out today, and it works great! Much better performance than I'm seeing on the demo site. That said, I would like to request one small tweak.

I think we should change the default settings in CommunityListService to only initially load 20 top level communities by default, instead of 50. When there's a total of 50 communities loaded at once, I'm still seeing some slowness, even with your PR applied. This setting seems to be controlled here: https://github.com/DSpace/dspace-angular/blob/main/src/app/community-list-page/community-list-service.ts#L104

Overall though, this looks good. I'd just request that one minor change before we merge this.

[tdonohue]

Thanks @MarieVerdonck ! This looks good now, merging!