Fix check for whether jq is installed in the system

[rchl]

The check for the existence of jq using shutil.which didn't really work since shutil.which does not ever throw so jq_exists have always been True, even if jq was missing on the system.

Fixed that and also:

• replace status message saying that JQ is missing with a dialog that also allows opening the JQ website (having a link in a status field wasn't a great idea since it was not possible to click it).
• remove unnecessary globals and just check if jq is available on every invocation. The caching of state is not necessary since this is never gonna be a performance issue. Also it would cache too hard, even after the user has changed the jq_binary in settings.