cascase deletes on Variants and Installations

Better support of cascading deletes:

• When deleting a Variant a dialog should show up if the user also wants to delete all the 'attached' device installation metadata (from all the devices)
• When deleting a PushAPP a dialog should show up if the user also wants to delete all the 'attached' variants, including all of its device installation metadata (from all the devices)

If the answer to the dialog is YES; a Cascading DELETE should be performed; Otherwise we keep the installation in the database - which allows some Admin to perform 'relocation' by hand...

Related ML thread:
http://aerogear-dev.1069024.n5.nabble.com/aerogear-dev-UnifiedPush-cascase-deletes-tt5461.html