Ability to white label and use a vanity URL with Rill Cloud

[AndrewRTsao]

Rather than using ui.rilldata.com, customers may want to white label / use a custom URL to access Rill Cloud. The approach right now would be to embed Rill Cloud dashboards as an iframe , but there could be reasons where customers may want a separate landing page entirely. In such a case, we should provide some level of admin overrides that customers can then use to implement a vanity URL with Rill Cloud.