consolidate scrapers

[Robert Jäschke]

A search for "/action/downloadCitation" reveals more than 15 scrapers that use this pattern to download the BibTeX metadata. It is likely that the corresponding sites all use the same backend software, namely Literatum (see the LiteratumScraper). This should be consolidated: if possible, all sites should be handled by one scraper (though this might result in a very complex scraper) or by a common superclass to share code.

[Robert Jäschke]

This is a good starting point to consolidate scrapers.

[Robert Jäschke]

refactored scrapers for sites using Literatum to extend the LiteratumScraper, fixes issue #2815

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