Reorganise flow-post-interaction link styling

Apply hover/click highlight to post interaction links. The light
buttons are currently a modification of .flow-post-interaction
.mw-ui-button. They should be implemented in core, which will be
addressed in bug 62744.

The current interaction links added by other extensions includes
Thanks, which is addressed in I3adb60d5b1c75500f8276cda8b8eaa2646bb1565.

Bug: 62556
Change-Id: If8b15f659e0191f84608c0781bf50fe7054ae661