clarify possibility to upload multiple problem zip files at once
Add test source exhibiting problem with compile time testdata access.
(cherry picked from commit 1d0264e838bd1cf91139838f3ef0293768e19f8f)
Use unprivileged user to compile program to make sure that it
doesn't have access to e.g. testdata files.

(cherry picked from commit 75a674efa45c9cbc82c0af5238be2f084bf6a25f)
Permit domjudge-run access to judging directory already at compile
time and revoke permissions dir after compiler-error.

Cherry picked from 08d69c155a7e6dd0e0dc0086be810bb384d43b66
Add changelog for compile security issue.