BUG: Subsite save SiteConfig overwrites MainSite SiteConfig (Issue #15)

Implemented the fix mentioned in the bug ticket within the subsites
settings extension (use current subsite id)
Merge pull request #88 from adrexia/siteconfig

BUG: Subsite save SiteConfig overwrites MainSite SiteConfig (Issue #15)
Change the preview iframe to use injected SubsiteID GET/POST params.

Use the CMS domain for fetching the preview content to avoid SSL
errors and single-origin violations on the iframe.
Merge pull request #90 from mateusz/previews-by-id

Change the preview iframe to use injected SubsiteID GET/POST params.
BUG Fixing "Add new" not adding the page to the correct subsite
Add custom menu-icon
Merge pull request #92 from adrexia/icon

Add custom menu-icon
$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They use all kinds of outdated APIs (in test system and CMS controllers),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.