gui: prompt does not work when backgrounded

[oliver-sanders]

This (and any other) prompt, will not work if the command is backgrounded:

cylc-uiserver/cylc/uiserver/scripts/gui.py

Lines 148 to 149 in def8d01

	ret = input(
	cparse('<yellow>Do you want to remove this file? (y/n):</yellow>'))

E.G:

cylc gui &

This works (because it's sending the text to the processes stdin, not the terminal):

cylc gui <<< y &

Ideally, we would fail the command in this situation, rather than issuing a prompt that the user can not answer.