Add an "always expand all sections" option

This adds an alpha mode option to Special:MobileOptions that controls whether
sections are always expanded on page load (previously they were expanded
only in sufficiently wide browser windows). The setting is stored in local
storage in order to benefit from existing page caches.

Bug: 45951
Change-Id: Ib5ab3af79e4cdd53c110ed1fbd0d91f5be5cc094