Fix 404 for docsite subpages when hard reloading app

[esimkowitz]

This adds a custom resolver to the docsite server to fall back to .html addresses when it gets a not found for a given path. This fixes an issue where subpages would return a 404 after a hard reload of the frontend. I also added an effect that will run on startup to resolve the latest docsite url and set it in the metadata, since the backend port changes on every run of the app. I also made it so that setting the default homepage in the webview widget will also unset any block-specific homepage.