full-mv-patch

[zachjesus]

Summary

• Move pg_trgm extension management out of the materialized view script into dedicated migration scripts (15/16/17).
• Rename 15_materialized_view.sql -> 18_materialized_view.sql so it runs after the extension is installed.

[cpeel]

@zachjesus - I've rebased this branch off of main. Because this PR doesn't yet have the updates in gutenberg_public.sql (because the MV hasn't been applied at ibiblio) I've moved it to a draft.