Warn the user when available RAM / memory is low

Originally created by @emmapeel on #7782 (Redmine)

From a user point of view, being warned when the computer is running low of RAM might be useful to understand why some software crashes or the computer gets slower or freezes, and take the needed action to remove stuff or close programs.

Marking this as a research ticket for the moment.

What others do

• Fedora enabled systemd-oomd (#19454 (closed)) in favor of the previously enabled earlyoom
  • According to the man page, systemd-oomd does not work well without swap ("Without swap, the system enters a livelocked state much more quickly and may prevent systemd-oomd from responding in a reasonable amount of time"). That should not be an issue since we have zram swap.
  • Longer-term, they want to consider https://gitlab.freedesktop.org/hadess/low-memory-monitor/ (citation needed).
• Fedora enabled zram (#5740 (closed)), and we did too