* 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
This is commonly requested by deployments with custom setups or those using SSO. Instead of having them all ship the same welcome.html with redirect code in it, we can offer a built-in redirect.
Note that this doesn't actually redirect and instead just replaces the view. This is to make the change less invasive as otherwise it involves changing the routing layers.
