Wait for moderator fails in some deployments

[jfederico]

Describe the bug Wait for moderator fails for guest users so when the moderator joins the guest or viewer keeps waiting in a loop.

Apparently this is only the case when GL was deployed in a sub-directory such as /b or /gl (like in demo or any standalone BBB server).

To Reproduce Steps to reproduce the behaviour:

1. Share a room link with a viewer. Go to the link as a viewer, type a name and click join. The spinning wheel shows up.
2. In a another browser, join the same room as moderator

Expected behavior The viewer should be brought to the room

Actual behavior The viewer keep waiting forever

Additional context There seems to be a problem with actioncable trying to reach root instead of the relative root from where the app is served