This allows to configure additional configuration files for Synapse. This way
secrets can be kept in a secure place on the file system without a need to go
through the Nix store.
The matrix-synapse user has `createHome = true;` which runs before the
`preStart` script, so the home directory will always exist and the block
will never execute.
Also don't include default path to keys in the configuration file,
because synapse will choke if it tries to open them before they
exist (even with `--generate-keys`).
We don't want to build all those things along with the manual, so that's
what the defaultText attribute is for.
Unfortunately a few of them were missing, so let's add them.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
