Extra checks frontend

[BramMeir]

closes #398

Extra checks frontend functionality:

• Option to add multiple extra checks when creating a project.
• Option to add/remove extra checks when updating a project.

Some little backend fixes

[Topvennie]

Nice one!

[DeLany123]

Super clean dialog field.
Some small comments:

• "Making logfile visible to students", I think it would be a little better to give more context to the logfile here, like "Making the extra logs of the docker container visible to the students" for example.

I think the different layout of the extra checks stands a bit to much out.