fix #5544: enlarge dropdown for external source import

[guillermo-escire]

References

Fixes #5544

Description

Enlarges the external source dropdown in the Import from External Sources page by increasing the initial page size from 5 to 20 and fixing the dropdown max-height CSS to allow proper scrolling.

Instructions for Reviewers

List of changes in this PR:

• Increased elementsPerPage from 5 to 20 in submission-import-external-searchbar.component.ts (both in ngOnInit and onScroll) so more sources are loaded initially.
• Updated .scrollable-menu max-height in submission-import-external-searchbar.component.scss from calc(var(--ds-dropdown-menu-max-height) / 2) to calc(var(--ds-dropdown-menu-max-height) * 2) and added overflow-y: auto to enable scrolling.
• Added/updated specs in submission-import-external-searchbar.component.spec.ts to reflect the new elementsPerPage value.

How to test:

1. Go to My DSpace
2. Click the import button (↵) and select "Publicación"
3. On the "Import a publication from an external source" page, click the source dropdown (e.g. "arXiv")
4. Verify the dropdown is larger and shows more than 5 options with scroll

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