Add v4.0.1 migration: fire acorn.user.tables.ready event for cross-pl…

…ugin support

Calendar (and any future dependent plugin) listens for this event to complete migrations
that were deferred because User was not installed at the time. Idempotent on re-run.

Add v4.0.1 migration: fire acorn.user.tables.ready event for cross-pl…

…ugin support

Calendar (and any future dependent plugin) listens for this event to complete migrations
that were deferred because User was not installed at the time. Idempotent on re-run.