Resolve "More group creation options" (1)

Closes #23 (closed)

What does this MR do (summary)?

• Students can now actually see the groups they can join.
• Merged the two existing group list pages into one.

Screenshots

What changes have you made (detailed)?

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.