[BUG] Fix remember current series

[BTWS2]

The plugin should remember the current series so you don't have to manually select it every time you make a new exercise #131.

When I make a new Dodona exercise the plugin doesn't remember the course and series when I want to create a new exercise. Are you able to reproduce this issue?

The plugin now always selects the first course from this academic year and the last (most recent) series.

The exercise sequence should be remembered as long as PyCharm is open. Usually a specific series of exercises is made in one lab. This way, the student does not have to select the correct series every time. Once PyCharm is restarted, the series may be forgotten.