db-migrations: prevent use of application code

[alxndrsn]

Mixing application code into database migrations removes assurances that the same migration will mutate the same postgresql data identically when upgrading from/to different versions of the application.

• add exceptions to existing use of application code in db migrations
• discourage future use of application code in db migrations by adding an eslint rule to error for local includes outside lib/model/migrations

[alxndrsn]

A low-impact example of this is shown at:

central-backend/test/integration/other/migrations.js

Lines 57 to 61 in 6324ebe

	// NOTE/TODO: figure out something else here D:
	// Skipping these migrations because after adding a new description
	// column to projects and forms, it is not possible to migrate part way
	// (before the new column) and populate the data when frames expect the
	// new column to exist.