* ci: add DB migration ordering/naming validation script
* ci: run DB migration validation on pull requests
* ci: don't reject non-migration .go files
* ci: fetch latest master before validating migration order
* ci: annotate the offending migration file on validation failure
* ci: gate Build on the migration check so a bad migration fails fast
* ci: reject migrations placed in a subdirectory of db/migrations
* ci: reword fetch-step comment to not hard-code the base branch name
