We also need to fix `--tags=register-user`. This tag is currently
defined in both the `matrix-synapse` and `matrix-dendrite` roles.

All the `matrix_s3_*` stuff happens in the `matrix-synapse` role.

If we are to have such S3 support for Dendrite, we should probably
extract it out of the `matrix-synapse` role (into a `matrix-s3` role or
`matrix-goofys`, etc.) and wire `matrix-dendrite` accordingly.

This may or may not be done in the future though. For now, I'm
cleaning things up in the `matrix-dendrite` role.

This brings dendrite.yaml up to date and cleans things up a bit.

We refrain from logging to files for all components,
because we rely on systemd-journald anyway.