Fix Caching Login Page

Fix Caching Login Page

- Check `canViewPage()` before adding page to full page cache in
  `PageCache::shouldAddToCache()`

Prevents issues where user without view permissions visits page and
caches the /login page. Then when permissions are changed or other
issues user sees /login page when should see actual page
Merge pull request #1172 from mkly/fix_full_page_caching_login_page

Fix Caching Login Page