Create blog entry for shared database support

[koppor]

See JabRef/jabref#1451 for the code.

• Automatic change push to the server via EventBus. (Support for all kinds of EntryEvents, see AddEventSystem jabref#1028)
• Full synchronization support for MySQL, PostgreSQL and Oracle
  (manual and semi-automated pull)
• Conflict avoidance by version control and Optimistic Offline Lock
• Connection loss handling
• Switch to offline working mode

One can test the feature using https://www.freemysqlhosting.net.

[mairdl]

@koppor on slack you asked for a help.jabref.org entry but here you ask for a blog entry, do you want them both or do you want us to create a blog entry that can be used for a help entry?

[koppor]

In the end, I want both. Based on the experiences of JabRef/www.jabref.org#58 and that the help page is the longer lasting asset, I stick with asking you for a help page.

[koppor]

The help file is available at http://help.jabref.org/en/SQLDatabase

[koppor]

Blocked by JabRef/jabref#2689

Maybe these issues should be solved before making advertisements for the feature: https://github.com/JabRef/jabref/issues?q=is%3Aopen+label%3Ashared-database