Completed system tests of leaderboards. Fixed bug where an empty leaderboard was visible on the 'create new exercise' page

Completed integration tests of likes

See merge request !8

Workout likes

See merge request !3

Leaderboards

See merge request !1

Merge branch 'leaderboards' of https://gitlab.stud.idi.ntnu.no/tmwang/tdt4242-base into leaderboards

Merge branch 'leaderboards' of https://gitlab.stud.idi.ntnu.no/tmwang/tdt4242-base into leaderboards