added GUI improvements to schedule, re #4421

app/controllers/calendar/schedule.php

app/views/calendar/schedule/_colour_selector.php

app/views/calendar/schedule/course_info.php

app/views/calendar/schedule/entry.php

app/views/calendar/schedule/settings.php

db/migrations/6.0.13_alter_schedule_table.php

db/migrations/6.0.35_new_schedule_improvements.php

lib/classes/calendar/Helper.php

lib/models/calendar/ScheduleEntry.php

resources/assets/stylesheets/scss/calendar.scss

resources/assets/stylesheets/scss/my_courses.scss

resources/assets/stylesheets/scss/schedule.scss

resources/assets/stylesheets/studip.scss