Add an option to not send read receipts.

Note that currently Riot clients use the read receipts to mark a room as
read. So disabling them will have the downside that the rooms will stay
as unread on Riot clients. It will also prevent synapse to dismiss
notifications if any.

See:
https://matrix.to/#/!DgvjtOljKujDBrxyHk:matrix.org/$150940327657oICsH:riot.ovh

Author: erdnaxeli

matrix.lua

@@ -1312,7 +1312,11 @@ function MatrixServer:SendReadMarker(room_id, event_id)
         postfields = {}
     }
     data.postfields['m.fully_read'] = event_id
-    data.postfields['m.read'] = event_id
+
+    if w.config_get_plugin('read_receipts') == 'on' then
+        data.postfields['m.read'] = event_id
+    end
+
     data.postfields = json.encode(data.postfields)
     http(url,
       data,
@@ -3326,6 +3330,7 @@ if w.register(SCRIPT_NAME, SCRIPT_AUTHOR, SCRIPT_VERSION, SCRIPT_LICENSE, SCRIPT
         encrypted_message_color = {'lightgreen', 'Print encrypted mesages with this color'},
         --olm_secret = {'', 'Password used to secure olm stores'},
         timeout = {'5', 'Time in seconds until a connection is assumed to be timed out'},
+        read_receipts = {'on', 'Send read receipts. Note that not sending them will prevent a room to be marked as read in Riot clients.'}
     }
     -- set default settings
     for option, value in pairs(settings) do