Merge branch 'feature/basket' into develop

Basket functionality is now available for all output methods: Instead of a
single TextGrid URI (referring to a TEI document or aggregation), you can also
specify a number of URIs, separated by commas. The result will contain all
referenced objects, and everything aggregated recursively from them, as well.

Known Limitations
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The metadata for the overall root object is rather minimalistic. For ZIP export, this only affects the generated filename (which will be <First_title>_etc.zip), for the other content types, this is true, as well.

There has not been much caching optimization for the basket case yet.