Graceful release merging

[kellnerd]

Currently every conflicting GTIN and/or track count makes the entire combined release lookup and merging fail. While this makes sense for lookups by GTIN where it is unclear which release the user wants to have, in many other cases that is quite clear:

• Lookup by a single release URL or identifier should merge into that release
• Release Actions are always for one specific MBID and should merge into the MB release

For these cases it would be more useful to drop all conflicting releases and still merge those which are compatible with the target release. Showing a warning which releases were dropped is enough.
Maybe the Release Actions page should even suggest to remove such external links from MB (unless they suffer from a provider which is not returning all tracks of the release).

[kellnerd]

Until this gets implemented, it is still possible to avoid the failure by disabling the conflicting providers. Although the Release Actions page has no provider checkboxes, this also works there because the accepted query parameters are mostly the same. (You can append the internal names of the enabled providers to the query parameters, for example &deezer.)