R1:86b08514abea

Merge TYPE_PROJECT_COLUMNS and TYPE_COLUMN transactions into a more general TYPE_COLUMNS transaction

Summary:
Ref T6027. We currently have two different transaction types:

- `TYPE_PROJECT_COLUMNS` does most of the work, but has a sort of weird structure and isn't really suitable for API use.
- `TYPE_COLUMN` is this weird, junk transaction which mostly just creates the other transaction.

Merge them into a single higher-level `TYPE_COLUMNS` transaction which works properly and has a sensible structure…