refactor(activity): migrate to PageShell + ColumnsLayout

Replace .activity-page wrapper + SidebarLayout with PageShell +
ColumnsLayout cols="events-sidebar". Delete the now-redundant
flex-chain CSS that PageShell owns. Adds .loading-state to the
visual mask list to handle the connections-mobile race between
the loading and loaded states.

e2e/visual/pages.spec.js

@@ -78,6 +78,7 @@ function commonMasks(page) {
     page.locator('.filter-bar'),
     page.locator('.skills-bar'),
     page.locator('.connections-section'),
+    page.locator('.loading-state'),
   ]
 }