Patch for issue setting subsite ID in session when editing in CMS, also allowed actions fix

The first commit is a patch for this issue: https://github.com/silverstripe/silverstripe-subsites/issues/93. It isn't a complete solution but it does fix the issue of subsite ID changing in the CMS when the preview is loaded. 

Second commit is related to allowed actions, now that actions are denied by default in controllers I needed to explicitly allow actions in the subsites gridfield detail form item request.

Its probably not ideal to have 2 separate issues referenced in this pull request and the first is a patch rather than a fix - merge at your discretion :-)