feat(mod_auth_token): add support for 'previd' query param

The 'previd' query parameter will be use to match user id of the session
being resumed when the smacks module and token authentication are
enabled in Prosody. Otherwise user gets new random id every time and
this doesn't work with the smacks module.

resources/prosody-plugins/mod_auth_token.lua

 local new_sasl = require "util.sasl".new;
 local sasl = require "util.sasl";
 local token_util = module:require "token/util".new(module);
+local sessions = prosody.full_sessions;
 
 -- no token configuration
 if token_util == nil then
 	if query ~= nil then
         local params = formdecode(query);
         session.auth_token = query and params.token or nil;
+        -- previd is used together with https://modules.prosody.im/mod_smacks.html
+        -- the param is used to find resumed session and re-use anonymous(random) user id
+        -- (see get_username_from_token)
+        session.previd = query and params.previd or nil;
 
         -- The room name and optional prefix from the bosh query
         session.jitsi_bosh_query_room = params.room;
 
         if (customUsername) then
             self.username = customUsername;
+        elseif (session.previd ~= nil) then
+            for _, session1 in pairs(sessions) do
+                if (session1.resumption_token == session.previd) then
+                    self.username = session1.username;
+                    break;
+                end
+        	end
         else
             self.username = message;
         end
 	local result, err, msg = self.profile.anonymous(self, username, self.realm);
 
 	if result == true then
-		self.username = username;
+		if (self.username == nil) then
+			self.username = username;
+		end
 		return "success";
 	else
-
 		return "failure", err, msg;
 	end
 end