RGB underglow support for JJ40, clean up redundant code in Mechmini keymap (#2196)

RGB underglow support for JJ40, clean up redundant code in Mechmini keymap (#2196)

* Cleanup Mechmini keymap. Once the custom RGB function is defined, there is no need to manually handle RGB code.

* Change default to KEYMAP_MIT, not KEYMAP_OFFSET

* Add custom RGB code for JJ40

* Reset Mechmini advertised power draw to 500. Will have to test actual maximum power draw later.

* RGB working on JJ40.

* Fix: saturation increase/decrease flipped

* Add new directory for my custom keymap with RGB keycodes

* Swap LAlt and LGUI

* Update JJ40 max power draw with measured value

* Update: fun40 rules.mk to enable underglow; earlier failed Travis CI