fixes #5249 chore(project): add some default reviewers for DB migrations (#5251)

Because:

* We'd like some elevated attention to PRs involving database migrations

This commit:

* Adds some folks to CODEOWNERS for changes to DB migrations

.github/CODEOWNERS

@@ -1 +1,6 @@
-app/tests @jrbenny35 @pdehaan
+app/tests @jrbenny35 @pdehaan
+app/experimenter/base/migrations @jaredlockhart @lmorchard
+app/experimenter/experiments/migrations @jaredlockhart @lmorchard
+app/experimenter/notifications/migrations @jaredlockhart @lmorchard
+app/experimenter/projects/migrations @jaredlockhart @lmorchard
+app/experimenter/reporting/migrations @jaredlockhart @lmorchard