WikimediaUI theme: Use WikimediaUI Base `size*` variables
WikimediaUI theme: Use WikimediaUI Base `size*` variables

Turn to the `@min-size*` vars into relative carrying values and use them
for `@size*` vars.
Also:
- Remove `@size-button-base` which hasn't been used.

Bug: T165650
Change-Id: Ic59e0c209aea8161cc625146d18a736529debd56

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui