ui: consider an admin command for installing new UI versions

[oliver-sanders]

Installing a new UI version is as simple as unpacking the tarball from GH releases into the appropriate directory.

Consider making this easier with an admin command or making it doable via pip install or conda install (would make the UI a Python package which is actually not too difficult).

Low priority, perhaps see how the UI development plays out, this may or may not be useful.

Questions:

1. Do we need to do this? Is there a demand for small UI updates independent of Cylc meta-package releases?
2. Command to install static files or Python package?

Pull requests welcome!