Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/3470
- https://github.com/hifi/heisenbridge/releases/tag/v1.15.0

During testing, it appears that Heisenbridge generated media URLs
that look like this: `{media_url}/_matrix/media/v3/download/DOMAIN/FILE_ID/FILE_NAME`.

This seems off. We were expecting `{media_url}/_heisenbridge/media/something`
(e.g. `https://matrix.DOMAIN/heisenbridge/_heisenbridge/media/something`, leading to its own media proxy),
but Heisenbridge still seems to be generating URLs destined for the homeserver's Media API.

Until we figure out why that is, `media_url` remains pointed to the homeserver URL (just like before),
so that the bot can continue operating like before.