Remap systemwide Caps + HJKL to arrow keys for Ubuntu 18.04 LTS (Bionic Beaver).
In file /usr/share/X11/xkb/symbols/us append in xkb_symbols "basic" scope before the closing bracket:
//hjkl
key <AC06> { [ h, H, Left ] };
key <AC07> { [ j, J, Down ] };
key <AC08> { [ k, K, Up ] };
key <AC09> { [ l, L, Right ] };
key { [ ISO_Level3_Shift ] };