Improve performance of RfC creation

[MrSerth]

Currently, creating an RfC is rather slow: It includes the following requests:

1. Create a new submission
2. Create the RfC (which will run all tests sequentially)
3. Run the code (execute the main file)

Right now, the modal allowing learners to post their question (as part of the RfC) is only closed once the second step completes. Still, it would be great to improve the performance of creating an RfC and decouple it more from the frontend.

This change should be rather easy once #2703 is finished.