Dendrite uses a lot of databases, but a single (`dendrite`) role, which
leads to `matrix_postgres_import_roles_to_ignore` being something like
`['dendrite', 'dendrite', 'dendrite', ...]` needlessly.

This leads to weird regexes being generated for
`matrix_postgres_import_roles_ignore_regex`.
It's not that it hurts, but it just looks odd.