cylc gui should use the hub (if available)

[datamel]

In #370, functionality was added so a uiserver launched by cylc gui would be reused by default on subsequent cylc gui commands.

cylc hub also launches a uiserver when the user visits the stated url. It would be ideal if cylc gui could detect (and open with) an already launched uiserver from the cylc hub command.

In order for implementation, we will need to add functionality for the site to configure the hub url. cylc gui logic will need updating to redirect to this hub if it is running. Authentication in this case will need consideration.

[datamel]

Note for implementation:

In hubapp.py we will need to set (and unset)the JUPYTER_RUNTIME_DIR environment variable. This will ensure the jp-server pid file gets saved in the correct folder so it can be discovered by cylc gui logic.

The pid file (if selected) will need its url adapting as the currently created file will direct to a 404.

[oliver-sanders]

Duplicate of #238