Add dilly/pletcher layout (#2880)

Add dilly/pletcher layout (#2880)

* Add pletcher keymap to dilly/keymaps

For the moment, this keymap just removes unneeded RGB keycodes, since
an iPad will cut the keyboard off if LEDs are turned on.

* Drop support for A_RSFT, add media and autoshift

* Lower USB_MAX_POWER_CONSUMPTION for dilly on iPad

* Document USB_MAX_POWER_CONSUMPTION

This config option is useful for limiting the requested power by, e.g.,
an iOS device. While the default value is 500, a much lower value--say,
50--can sufficiently power a small keyboard without LEDs.