Use & enforce snake_case naming convention on config.json settings (#8062)

* Document and support the established naming convention for config opts

This change:
* Rename `ConfigOptions` to `IConfigOptions` to match code convention/style, plus move it to a dedicated file
* Update comments and surrounding documentation
* Define every single documented option (from element-web's config.md)
* Enable a linter to enforce the convention
* Invent a translation layer for a different change to use
* No attempt to fix build errors from doing this (at this stage)

* Add demo of lint rule in action

* Fix all obvious instances of SdkConfig case conflicts

* Fix tests to use SdkConfig directly

* Add docs to make unset() calling safer

* Appease the linter

* Update documentation to match snake_case_config

* Fix more instances of square brackets off SdkConfig

test/components/structures/auth/Registration-test.js

@@ -34,7 +34,7 @@ describe('Registration', function() {
     let parentDiv;
 
     beforeEach(function() {
-        jest.spyOn(SdkConfig, "get").mockReturnValue({
+        SdkConfig.put({
             ...DEFAULTS,
             disable_custom_urls: true,
         });
@@ -46,6 +46,7 @@ describe('Registration', function() {
     afterEach(function() {
         ReactDOM.unmountComponentAtNode(parentDiv);
         parentDiv.remove();
+        SdkConfig.unset(); // we touch the config, so clean up
     });
 
     function render() {