Implement integration test, improve performance on large exams

.gitlab-ci.yml

CMakeLists.txt

Dockerfile

src/CMakeLists.txt

src/data/nodes/changed_max_points.cpp

src/file_io/d2r_parser.hpp

src/view/gui/custom_double_spin_box.cpp

src/view/gui/custom_double_spin_box.hpp

src/view/main_window/export_rwthonline.cpp

src/view/main_window/export_rwthonline.hpp

src/view/main_window/grade_tab.hpp

src/view/main_window/main_window.hpp

src/view/main_window/point_histogram.hpp

src/view/main_window/students_tab.cpp

test/CMakeLists.txt

test/data/nodes/dynexite_exam_test.cpp

test/data/nodes/first_try_test.cpp

test/data/nodes/student_data_test.cpp

test/data/nodes/user_grade_boundaries_test.cpp

test/data/nodes/user_grade_config_test.cpp