Fix all code that is using creatability as a proxy for persistability

[akhenry]

Summary

Some of our code is using the creatable property of object types to determine whether an object can be persisted. Creatability and persistability are two different things. The creatable property only tells you whether or not an object type appears in the Create menu and nothing else. An overlay plot can be created from the Create menu, but it can also be exposed as static JSON in the tree. Using creatability to determine whether the object can be persisted in this case will give a false impression, and will result in persistence errors if any mutation or persistence is attempted.

We need to search our code base for instances where the .creatable attribute is being used to determine persistability, and replace it with a call to openmct.object.isPersistable(domainObject).

Here is an example of an instance where creatability is being used instead of persistability:
#4149 (comment)

This is of fairly marginal impact right now, but it will soon become critical when we start building change-controlled display layouts and exposing them as non-editable objects in the tree.

Impact Check List

• Data loss or misrepresented data?
• Regression? Did this used to work or has it always been broken?
• Is there a workaround available?
• Does this impact a critical component?
• Is this just a visual bug?

[jvigliotta]

Testing

• make sure you can't select dictionary items/folders as destinations from the create menu
• make sure Duplicate, Move, Link, Remove, ExportAsJSON, ImportAsJSON actions are working properly
• try to edit or modify dictionary items or other non-editable items (Plans folder...)

[nikhilmandlik]

able edit properties for plans inside a Viper Plans and use another plan json.

[unlikelyzero]

covered by https://github.com/nasa/openmct/blob/master/e2e/tests/persistence/persistability.e2e.spec.js