add and use DatabaseConnectionRestService to do all actions related to DatabaseConnection
Added/moved tests for DatabaseConnectionResource
Merge remote-tracking branch 'origin/DatabaseConnectionRestService' into delete_algorithms

Conflicts:
	frontend/src/test/java/de/metanome/frontend/client/TestDatabaseHelperService.java
	frontend/src/test/java/de/metanome/frontend/client/TestDatabaseHelperServiceAsync.java
	frontend/src/test/java/de/metanome/frontend/client/TestDatabaseHelperServiceImpl.java
	frontend/src/test/java/de/metanome/frontend/client/TestHelper.java