Logical deletion for Activity records

Implementation of https://github.com/maiera/gde-app/issues/99

Activity records now how a boolean field `deleted`
Calling `activity_record.delete` will set this field to `True` and store the record.
To undelete you call `activity_record.update` (with full AR information, same as you already do when saving entities from the frontend) and `deleted` set to `False`

`activity_record.list` by default only returns items with `deleted=False`

To get all activity_records you can use `includeDeleted=true` as query parameter.

To get only the deleted items you can use `includeDeleted=true&deleted=true`

When going live we have to set `deleted` to `False` for all existing entities in the datastore otherwise the result of activity_record.list would be empty.

I also changed the gplus update task to automatically skip posts if they are associated with a deleted activity_record.