Finish hooking up the frontend from {T377575} to the [[ https://www.mediawiki.org/wiki/Community_Configuration | community configuration ]] backend storage. There are some unknowns eg. with our customized Codex interface, but the configuration itself for backlink symbol set is a single string.
* [ ] Define backlink configuration in `CiteSchema.php`. (Remove unused custom group config.)
* Root schema should be an Object, to leave space for additional features to be configured.
* Backlink configuration is a String naming the current alphabet.
* Backlink config defaults to some kind of `undefined`, which is a clear signal to use the legacy.
* Discuss: do we allow "unsetting" to revert to legacy backlinks? It would offer a soft migration. If we do so, plan to make it a temporary stage, and give the unset value an explicit magic name, eg `REVERTED` to be distinguished from the undefined initial state.
* [ ] Implement a new provider in extension.json `CommunityConfigurationProviders`. A wikipage json store like in https://www.mediawiki.org/wiki/Extension:CommunityConfiguration/Technical_documentation#Registering_a_new_provider is probably ideal.
* [ ] Needs to show up as a subpage like `Special:CommunityConfiguration/Cite` . Stop publishing as `Special:CiteConfiguration`, if possible.
* [ ] Still hidden behind a feature flag `$wgCiteCommunityConfiguration === true`
* [ ] Load stored configurations into the UI, save changes back to the store.
Out of scope:
* Custom groups, which are configured by a variable-length array of nested config. Remove unused custom group config from CiteSchema.php