Allow the front end to add rows to a table with a PK column that has no dynamic default

[seancolsen]

Current behavior

• If the PK column in a table has a dynamic default, then the user is able to add new rows just fine.

• If the PK column in a table has null as its default, then the user gets stuck in this state when attempting to add a new row

  

  Here, the front end is performing client-side validation before sending an API request to create the record, and seeing that the id column defaults to null but has a not-NULL constraint (by virtue of having a primary key). The front end also prohibits editing PK cells. So the user is stuck without the ability to enter a new value into the id cell.

Desired behavior

• The user can enter a new value into the PK cell when creating the record.
• After the record is created, the user can no longer edit the PK cell value.

[pavish]

This seems to be a duplicate of #1356. I'm closing this in favour of that one.