Add devices filter to list untagged devices

[lawik]

If all deployments are tagged it becomes relevant to add tags to new devices. Finding devices which are untagged has been impossible using the current set of filters.

This adds a filter for Untagged devices.

[lawik]

Both flip it and reverse it? Absolute madman :)

Not sure the inverse is a large improvement.

[joshk]

for me only_untagged doesn't read well. has_tags feels nicer.

[lawik]

I prefer how that reads but not what it implies for defaults. What's the default? has_tags: true, implies showing only things with tags has_tags: false, implies showing only things without tags has_tags: ??, implies left blank

I guess we could do empty string to blank it as with other defaults but I think I prefer a boolean. How about has_no_tags?

[joshk]

+1 on has_no_tags or empty_tags