Switch link widget to extend apos-link-widgets

[abea]

https://github.com/apostrophecms/apostrophe-open-museum/blob/23312897a4113a3ea7e9e3d9097ec2b4ce613b89/lib/modules/link-widgets/lib/schema.js#L1-L64

Any objection to changing this to extending apos-link-widgets: https://github.com/apostrophecms/apostrophe-link-widgets/blob/master/index.js? Or maybe not extend, but use the existing schema from that module. The schema names are all the same, plus some in apos-link-widgets. We could turn the Open Museum link widgets into an example of defining the schema by pulling it out of an existing module. It also lets us highlight more ready-made modules.

CC @stuartromanek @boutell @grdunn

[stuartromanek]

No objection, as long as testing the transition is all good and works