Bugfix and Enhancements

Fixed the issue that invalid clients can publish will message. (emqttd#1230)

Fixed Dashboard showing no stats data (emqttd#1263)

Fixed a rare occurred building failure (emqttd#1284)

Support Persistence Logs for longer time (emqttd#1275)

Fix for users APIs (emqttd#1289)

Changed passwd_hash/2 function's return type (emqttd#1289)

emq-dashboard Plugin (emq-dashboard#154)

Improved the Dashboard Interface of Monitoring/Management/Tools.

Allow switching dashboard themes.

Supoort both EN and CN languages.