BUG Move the SubsiteList PJAX request to a dedicated Controller.

Currently the request cannot be made if one doesn't have access to the
SubsiteAdmin section, which often happens with subsite-specific admins.
API Refactor to always redirect to accessible Admin location.

Tries to find an accessible section in the current site, falls back to
searching across all sites and all sections.

Also adds more powerful and generic functionss:
Subsites::all_sites - get the full list
Subsites::all_accessible_sites - get Member accessible list
LeftAndMainExtension::sectionSites - get section-specific list
Fix the test coverage for the subsite access changes.
Merge pull request #115 from mateusz/admin-access

Fix CMS Admin access issues
Hide subsites dropdown for collapsed sidebar

Its cut off otherwise, and not really operational.
This is consistent with hiding the "hi <user>" string
as default CMS behaviour.