Provide channel root url and relative url for current page to extensions

The Sitemorse extension runs an analysis on the current page and shows the results in the side panel. To enable the extension to run it's analysis on the preview version of the page, we can do the following:

1. Configure a separate preview mount for the channel,
2. Pass the preview mount name to the extension in the config string
3. Have the extension modify the current page url to "inject" the preview name

There is (at least) one problem with this approach, which is that the extension needs additional information to figure out where to inject the preview name into the url because it can't know the channel context path. To fix this problem, we need to provide the extension with the root url of the channel, or the relative url of the current page, or both.