Series activities json should be sorted correctly

[jorg-vr]

Currently these activities are sorted by popularity. Expected behaviour would be for these activities to be sorted by series order.

This results in incorrect sorting in plugins

eg: https://dodona.ugent.be/nl/series/2518/activities.json

Potential cause: #4222

Potential solution:

Apply default sort only when no series is present (Line 71)
Instead of here: https://github.com/dodona-edu/dodona/pull/4222/files#diff-6c3192e53b5a2e74e2fe29782dce058fe0398676d09068812b92c40d88625755R80