Guest share block domains should have input validation in admin panel

[mrow4a]

per @phil-davis comment:
The string value is supposed to be a comma-separated list. But I can put in spaces between domain names, and I can enter any characters at all (that do not look like domain names)

We could:

do some easy automatic formatting (e.g. replace whitespace with commas, trim whitespace at beginning and end)
add some text to say that a comma-separated list of domain names is expected to be entered
(optional?) process the domain names through some "reasonable" validator to confirm that they are "valid" (the validator needs to accept less usual things like an IP address, "localhost" etc so that test/development installations can try local things)
I tested the code manually and it is working - any existing value is displayed, any change is correctly saved. I created issue #496 for an automated test and put it on our QA board. There is no need to block merging here.

ref.

guests/js/guests.js

Lines 124 to 129 in 69e01eb

	var saveShareBlockDomains = function () {
	// FIXME: do validations here to make sure valid input is passed
	// and valid domains
	config.shareBlockDomains = $guestSharingBlockDomains.val().trim();
	saveConfig();
	}

[mrow4a]

FYI @pmaier1