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

src/components/views/auth/CountryDropdown.tsx

@@ -59,7 +59,7 @@ export default class CountryDropdown extends React.Component<IProps, IState> {
         super(props);
 
         let defaultCountry: PhoneNumberCountryDefinition = COUNTRIES[0];
-        const defaultCountryCode = SdkConfig.get()["defaultCountryCode"];
+        const defaultCountryCode = SdkConfig.get("default_country_code");
         if (defaultCountryCode) {
             const country = COUNTRIES.find(c => c.iso2 === defaultCountryCode.toUpperCase());
             if (country) defaultCountry = country;