Support for moving data from other fields into a streamfield

[sandilsranasinghe]

There might be instances where it is needed to move data from another model field into a streamfield. For example: turn an inline panel into a streamfield.

This could further extend to even moving data from one streamfield to another.

One potential way to go about this would be to give operations access to the model (to read the required from the other model field), so that custom operations could then be written for the data migration.