Custom population's associated programme -feature, and minor fixes & improvements

Custom population: The search now has a field for associating the population with a study programme. This allows the use of personal study plan filter on that population.

Trends-view: User can use only the calendar for picking date

Several views: Students > General tab > Study programme column: Logic changed. Now if the population is associated with a programme, the column is titled "Other programmes" and will show newest programme other than the associated one.