Remapping Caps Lock to Escape in Ubuntu 18.04 bionic
Found an option by installing Gnome Tweaks.
sudo apt install gnome-tweaks
Then open Tweaks, and click:
Keyboard & Mouse -> Additional Layout Options -> Caps Lock key behavior.
(On older versions of Ubuntu, you would need to install gnome-tweak-tool
instead.)
In Ubuntu MATE 18.04, launch Keyboard
and go to Layouts > Options > Caps Lock behaviour
to customise Caps Lock (as an additional escape key, disable entirely, and other options).