Update admin color scheme using the REST API

[rmccue]

Reasonably simple admin-ajax call (notably, with no significant filters) that can be replaced with a call to /wp/v2/users. Patch attached.

One thing we can't do is provide an enum of valid color schemes, since historically these have only been registered on admin_init (including in core). This also applies to the sanitization callback on the meta key, which can only remove invalid characters, not validate the scheme is valid.

[rmccue]

Use /wp/v2/users/me for changing color scheme