Add none option to field source

I feel that this might behave strange if one saves with none and then tries to do any operations that depend on the field. I am pretty sure that would throw fatals at some point.

#2625854: Provide default source_field when creating new media entity bundles might be better solution. It is already implemented in Video embed field module, which implements plugin for embeddable videos. We would just need to generalize what is already there.