Enable to set activity icon outline colors

[lucaboesch]

In Moodle 4.4 activity icons will be shown as outlines.

The colors of those outlines will be set not with RGB hex values but like such:

invert(33%) sepia(8%) saturate(2138%) hue-rotate(227deg) brightness(92%) contrast(82%) !default;

This issue is about not forgetting to create settings to be able to modify those values.