Get filtered submissions

What does this MR do (summary)?

Adds the functionality to retrieve submissions from Core based on a filter for the (group) name and/ or if they are the latest submissions.

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.