chore: Relocate frontend migrations

Created by: efritz

https://github.com/sourcegraph/sourcegraph/issues/13885 requires that we duplicate the migration infrastructure used for the frontend so that we can use it independently but symmetrically in the code intel database.

This moves the frontend migrations so that we can make some room for additional migration paths. This is only split out to minimize the diff for later PRs which should not have lots of renames grabbing attention.