Work-in-progress limits per column

Columns can now carry a work-in-progress limit. Set one, and the column shows its count against the limit and flags when it is over.

The more interesting half of this release is what happens when a column cannot have one. Where the limit comes from depends on how the board is defined, and on a provider-driven board the answer is sometimes "not from here". Previously that just looked like a column with no limit, which is indistinguishable from a column nobody set a limit on.

Now the board explains why the limit is missing and where it would have to be set instead. Same principle as the non-label columns: an absence you cannot account for is worse than a message.