add Vips as CorePlugin, re #4258

app/views/vips/sheets/list_assignments_list.php

app/views/vips/sheets/list_assignments_stud.php

app/views/vips/sheets/list_exercises.php

app/views/vips/sheets/move_assignments_dialog.php

app/views/vips/sheets/move_exercises_dialog.php

app/views/vips/sheets/print_assignment.php

app/views/vips/sheets/print_assignments.php

app/views/vips/sheets/print_layout.php

app/views/vips/sheets/show_assignment.php

app/views/vips/sheets/show_exercise.php

app/views/vips/sheets/show_exercise_link.php

app/views/vips/sheets/start_assignment_dialog.php

app/views/vips/solutions/assignment_solutions.php

app/views/vips/solutions/assignments.php

app/views/vips/solutions/assignments_list.php

app/views/vips/solutions/assignments_list_student.php

app/views/vips/solutions/autocorrect_dialog.php

app/views/vips/solutions/edit_assignment_attempt.php

app/views/vips/solutions/edit_group_dialog.php

app/views/vips/solutions/edit_solution.php