Administration: Add extended datasource configuration

[bk-viadee]

(Relational) datasources should be configured via a standard properties-file (Spring Boot style).

• check prerequisites for that
• make SQL-Statements/scripts independent of DB-type, e.g. user Spring Data to generate generic DDL-Statements
• add tutorial for simple H2 file database

[sdibernardo]

Flyway migration seems to work fine for now, but another bug boggles my mind..

Caused by: java.lang.IllegalStateException: Multiple representations of the same entity [com.viadee.sonarQuest.entities.Developer#1] are being merged. Detached: [com.viadee.sonarQuest.entities.Developer@6c303cdc]; Detached: [com.viadee.sonarQuest.entities.Developer@7ec00f98]

This is due to the cascading annotations in the models and I cant seem to get rid of this .. Some input?

[sdibernardo]

Found and fixed the error