Support for OPDS "catalogs" feed

[danielweck]

I've just updated the JSON Schema, and I realised that support for is missing in Thorium.
See: opds-community/drafts#38

Test URLs:

• http://libraries.aldiko.com/home.json
• https://libraryregistry.librarysimplified.org/libraries

[danielweck]

https://libraryregistry.librarysimplified.org/libraries

=>

http://readium2.herokuapp.com/opds-v2-browse/https%3A%2F%2Flibraryregistry.librarysimplified.org%2Flibraries

[danielweck]

http://libraries.aldiko.com/home.json

=>

http://readium2.herokuapp.com/opds-v2-browse/http%3A%2F%2Flibraries.aldiko.com%2Fhome.json

...for example click on the "france" __href__ link:

https://libraries.aldiko.com/countries/france.json

=>

http://readium2.herokuapp.com/opds-v2-browse/https%3A%2F%2Flibraries.aldiko.com%2Fcountries%2Ffrance.json

(note that the "Canada" link is also a navigation feed, which exposes a selection of catalogs feeds (one additional level of indirection)

[danielweck]

Related: #274 (comment)

[panaC]

https://github.com/readium/r2-opds-js/blob/e410866da1483632c756d9f3a3f1f09bb1666bbd/src/opds/opds2/opds2.ts#L153

[panaC]

with : https://libraryregistry.librarysimplified.org/libraries

@llemeurfr do you agree with that ?
the feed contains an array of catalogs

[llemeurfr]

@panaC I think that what is displayed in your screenshot is of no use for end users.

If we want to support an OPDS "catalogs" structure in the Catalogs screen of Thorium, the goal must be to simplify the choice of one or more OPDS feeds inside such a set of feeds. The issue here is that entering the URL of the OPDS "catalogs" in a text field is a nerd thing (we see that with the current requirement to enter the URL of an OPDS catalog in Thorium).
A discussion on the best way to do this in opds-community/drafts#47 (comment).

Once the OPDS "catalogs" URL is known from Thorium, we'd like to see an "OPDS feed picker" opening and be able to navigate into the (hierarchical) set of OPDS feeds to select one or more feeds, automatically added to Thorium. Aldiko Next offers just that, via the "Add" button in their "catalogs" screen.