separate media and database paths

The primary motivation for this change is to support databases that
don't take a path, e.g. out of process databases.

This configuration structure leaves the door open for other media
storage mechanisms in the future, such as S3.

It's also structured to avoid `#[serde(flatten)]` so that we can use
`#[serde(deny_unknown_fields)]`.

tests/integrations/fixtures/check_config/minimal-valid.toml

@@ -5,4 +5,8 @@ client.base_url = "https://matrix.example.com"
 
 [database]
 backend = "rocksdb"
-path = "/var/lib/grapevine"
+path = "/var/lib/grapevine/database"
+
+[media.backend]
+type = "filesystem"
+path = "/var/lib/grapevine/media"

tests/integrations/fixtures/check_config/valid.toml

@@ -10,7 +10,11 @@ client.base_url = "https://matrix.example.com"
 
 [database]
 backend = "rocksdb"
-path = "/var/lib/grapevine"
+path = "/var/lib/grapevine/database"
+
+[media.backend]
+type = "filesystem"
+path = "/var/lib/grapevine/media"
 
 [federation]
 enable = true