Handling prohibitively long IIIF imports

[rsimon]

Case in point:

https://iiif.archive.org/iiif/bim_eighteenth-century_a-new-general-collection_1745_4/manifest.json
https://iiif.archive.org/iiif/descriptionofemp00duha/manifest.json

Both of these manifests:

• have hundreds of pages
• reference annotations via an external JSON file for (almost) every page
• are served from a slow server

Because IMMARKUS fetches all annotations during import, these manifests could take hours to load.

How to best deal with this?

1. Add support for "progressive" loading? I.e. don't import annotations until the user actually views a page? In this case we'd have to show a kind of "indeterminate" annotation count in the overview.
2. Introduce a heuristic (>20 external annotation links?) after which we prompt the user whether they want to skip importing annotations

2nd option is likely the quicker fix for now.