Improve Grafana dashboards and documentation

• Improve readme and add examples for file based configuration
• Add tag name "James" for each file (good to distinguish if you have other non-James dashboards running in the same Grafana server)
• Fix elasticsearch version
• Use unified data source name (DS_JAMES_ES) in all scripts (currently some dashboards like Imap_board and JMAP_board use DS_ELASTICSEARCH instead as datasource name)

I would like to work on this