Entityreference Selectall

Add a 'Select all options' checkbox to entityreferences.
This also refers future referring entities.

Only works on entityreference fields with a unlimited cardinality.
Does not work with core term reference fields.

Example:

You have Node A, with a entityreference field to Vocabulary B, which contains 3 terms.
When creating Node A, you selected the 'select all' checkbox.
This saves the entityreference field with all 3 terms.

When later on, a new term is created, we resave all our Nodes that have that 'select all' checkbox checked. For scalability, this is done with batch API.

Thanks to 'Digipolis Ghent' for the project time.