Merge tables which are connected by a Foreign Key relationship.

[mathemancer]

Problem

Perhaps a user has split a table using the functionality proposed in #67 , but they've done something wrong, or they've thought of a better set of columns to extract from the original table. They may want to put the tables back together (not just be able to view them together, but actually merge them under the hood).

Proposed solution

We should give them the ability to merge the tables back together to recover the previous state of their data.