$allowed_actions for GridField subclass

Taken from https://github.com/silverstripe/silverstripe-subsites/pull/94,
thanks @frankmullenger
BUGFIX: Session var for active subsite out of sync with current subsite. Refs silverstripe/silverstripe-subsites#93.
Fixed JS indentation
3.1 allowed_actions fixes for SubsitesTreeDropdownField
Removed disabled tests (#29)

They use all kinds of outdated APIs (in test system and CMS controllers),
and the function rename makes it unclear that they're actually disabled.
These kinds of tests should be performed through Behat anyway.
Less intrusive doSave() overwrite of GridFieldDetailForm

Fixes issues with valiation errors not showing due
to lack of PjaxResponseNegotiator support (parent implementation has changed).
Require subsite title (fixes #26)
Use correct jQuery variable
Change dependency versions to work for 3.1.0-rc1.
API: Subsite support for menu of cms (hide admins that don't declare support)  (fixes #101 and #89 )

* Hide admins without subsite support from subsites menu
* Add subsite support to default site areas
* Enable reloading of subsites switcher dropdown when navigating the
site, and when editing subsite areas

API Fix parallel pjax menu fetching for subsites.
- thanks Mateusz!

Delint LeftAndMain_Subsites.js
Merge pull request #104 from adrexia/subsites-ui

API: Subsite support for menu of cms (hides admins that don't declare support) (fixes #101 and #89 )
Added test case for subsites virtual page onAfterWrite issue, plus changed get_from_all_subsites method to immediately eval and return an ArrayList, instead of lazy eval DataList. Fixes #106
Update Documentation

* Updated graphics
* Updated information around global dropdown
* Added Information about supporting subsites in modeladmins
* Changed 'working.md' to 'working_with.md', for clearer menu naming
Better fix for #106 using DataQuery queryParams. Thanks simon_w
Merge pull request #107 from spronkey/106-get-from-all-subsites

Fix for issue #106 get_from_all_subsites to force immediate eval instead of lazy with DataList
Merge pull request #108 from adrexia/docs

Update Documentation
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