Resolve "When switching lab from random with prio to fcfs, students stay on pending"

What does this mr do?

Users can now change a capacity lab from random to fcfs. When this happens, the old selection will be removed and new requests will be selected based on their creation time.

The user will be notified that they are changing the selection procedure and that thus the selection might change.

Screenshots

before

after

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide. Closes #522 (closed)