Option to hide the clock on the Breeze-themed lock and login screens

This issue collects all the requisite merge requests in one place.

See !2365 (comment 996822) for explanations:

Author:

It's a political choice to align my life with rhythms of the world around me rather than the arbitrary impositions of clocks; to pay attention to what's actually happening around me, rather than what I'm normatively expected to be doing at any given time. There's a lot more to say than would be appropriate for a comment on a code review, so I'll link to a video essay exploring the topic in more depth.

I do experience stress when seeing clocks, especially when that's done without me having any choice over whether to look at the clock. It makes me think about the time that has passed, and fret about my plans for the future, rather than experiencing the present as it is. Since I removed the default clock widget from the panel (around a year ago), it has been much nicer to use my computer. I no longer get stuck in anxious loops sitting at my computer for far too long, glancing at the clock and telling myself "I'll stop and do something else in 20 minutes" and repeating that in 20 minutes until the sun comes up. I would like to have a solution to getting rid of the clocks on the lock and login screens, so I don't have to choose between using my computer and not looking at clocks. I want to be in full control of when I choose to engage with clock time, not have it forced upon me constantly. So if you want to consider it an accessibility issue, sure I guess.

Akseli:

Another possible argument for adding this setting:

• Some people have not-very-good monitors that still can get burn-in. Clock is very stationary piece, so it can cause this burn-in.

Nate:

The new option text is also much more comprehensible than the old one. I think it's easier for the average user to understand the actual clock visibility conditions now.