Check schema references when a change set publishes #2604

Part of #2415 (F-SCH). A follow-up from the review of #2520 (SCH-05).

A content type write checks its references against what each type is heading to: every draft an open set proposes, or the published version while there is none (SCH-04). Sets publish one at a time, though, so a write-time check cannot see the order they will publish in. Two cases still leave a live reference to a field the live target lacks:

• Set A adds logo to brand. Set B adds a page field that references brand.logo, which is accepted because the open draft of brand has it. Then B publishes before A, or A is rejected.
• A set that adds logo publishes, and a later set that references it publishes too. Reverting the first set takes logo away again (Add change set revert #2511).

Scope

• When a set publishes, check the references against what will be live once it has: those of each schema item the set makes live, and those of every live type that points at an item the set changes. On a failure, nothing is published and the answer names each dangling reference, as the write-time checks do (reference_field_unknown, reference_target_not_entity, field_referenced).
• A revert publishes like any other set, so it gets the same check.
• Let a set drop a field together with the last reference to it. Today the write-time check (field_referenced) still counts the referencing type's published version when the same set's draft of that type removes the reference. With the check at publish in place, the write-time check can trust that draft.

Out of scope

• References between entries (F-CNT F-CNT: Entries with versions, states and entity references #2416).
• Telling the author at write time which set a referenced field depends on. The desk can show that when it reviews schema (F-DSK F-DSK: Review desk as the home screen #2422).

Acceptance

• In both cases above, the publish that would leave the reference dangling fails. Nothing is published, and the error names the field and the type that references it.
• A set that adds a field and a reference to it publishes, and so does a set that drops a field together with the last reference to it.
• All of the above passes on SQLite and PostgreSQL.