Currently, we only check constraints on the statement main snak. We should also check them on qualifiers and on references.
- [x] Get new API format ready: {T174544}
- [x] [Add context for qualifiers](https://gerrit.wikimedia.org/r/#/c/373322/)
- [x] [Add context for references](https://gerrit.wikimedia.org/r/374995)
- [x] Update some constraint checkers (e. g. “conflicts with” probably conflicts with other qualifiers?) – not a blocker due to T175570, the various related tasks can be done later
- [x] [Add config for checking constraints on qualifiers and references](https://gerrit.wikimedia.org/r/#/c/376290/) ~~(might be the same as the one for switching to the new API format)~~
- [x] [Add support for checking constraints on qualifiers and references to extension](https://gerrit.wikimedia.org/r/#/c/379746/)
- [x] [Add support for checking constraints on qualifiers and references to gadget](https://gerrit.wikimedia.org/r/#/c/379747/)
- [x] Enable config on Wikidata: {T176863}
- [x] [Default config for new API format to `true`](https://gerrit.wikimedia.org/r/385387)
- [x] [Remove support for old API format (and config setting)](https://gerrit.wikimedia.org/r/385388)
- [ ] Default config for checking constraints on qualifiers and references to `true`
- …