matrix: Enable retention policy and all the suggested config

This gives us a maximum history of 1 year, configurable per-room down to
1 day.

roles/matrix/templates/homeserver.yaml.j2

@@ -334,15 +334,15 @@ retention:
   # The message retention policies feature is disabled by default. Uncomment the
   # following line to enable it.
   #
-  #enabled: true
+  enabled: true
 
   # Default retention policy. If set, Synapse will apply it to rooms that lack the
   # 'm.room.retention' state event. Currently, the value of 'min_lifetime' doesn't
   # matter much because Synapse doesn't take it into account yet.
   #
-  #default_policy:
-  #  min_lifetime: 1d
-  #  max_lifetime: 1y
+  default_policy:
+    min_lifetime: 1d
+    max_lifetime: 1y
 
   # Retention policy limits. If set, a user won't be able to send a
   # 'm.room.retention' event which features a 'min_lifetime' or a 'max_lifetime'
@@ -350,8 +350,8 @@ retention:
   # in which server admins can make sure every federating server applies the same
   # rules.
   #
-  #allowed_lifetime_min: 1d
-  #allowed_lifetime_max: 1y
+  allowed_lifetime_min: 1d
+  allowed_lifetime_max: 1y
 
   # Server admins can define the settings of the background jobs purging the
   # events which lifetime has expired under the 'purge_jobs' section.
@@ -374,13 +374,13 @@ retention:
   # that purge to be performed by a job that's iterating over every room it knows,
   # which would be quite heavy on the server.
   #
-  #purge_jobs:
-  #  - shortest_max_lifetime: 1d
-  #    longest_max_lifetime: 3d
-  #    interval: 5m:
-  #  - shortest_max_lifetime: 3d
-  #    longest_max_lifetime: 1y
-  #    interval: 24h
+  purge_jobs:
+    - shortest_max_lifetime: 1d
+      longest_max_lifetime: 3d
+      interval: 5m:
+    - shortest_max_lifetime: 3d
+      longest_max_lifetime: 1y
+      interval: 24h
 
 
 ## TLS ##