WIP 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, among other things:
- this squeezes a button on top of another button (100% wide expand/collapse button)
- "pin" button is actually faked ToggleButtonWidget which does not have its PHP counterpart yet
- "pin" icon SVG could be optimized even further but some part gets lost when doing full optimization with svgo. This should be investigated and the icon possibly changed.
- it might be also worth considering if the icon should be add to OO's icon set.
- browser tests for all pin/expand combinations