Add button to have revision slider expanded automatically
This adds a button which allows a user to make revision slider
automatically expand on each diff page (disabled by default).
User's choice is stored as a hidden user preference.
This is a bit hacky as it squeezes a button on top
of another button (100% wide expand/collapse button).
This also adjusts styles of ToggleButtonWidget so
the button looks more like a frameless button, although
it is created as a framed one (to have inverted behaviour
when button is in its "on" state).
The button only gets visible when Resource Loader finishes
loading JS and CSS to avoid button jumping around the top
of slider window before "right" styles are applied.