Boost courses page

#5437 
First change, changing semester filter block performance by using db_select() gives 33% performance boost when testing performance of `economics/courses` page.
Before:
Time per request:       2989.085 [ms] (mean)
After:
Time per request:       1993.918 [ms] (mean)

Second change, using fields in view instead of node rendering, gives another boost.
After (both changes):
Time per request:       1339.661 [ms] (mean)

Bottom line - 55% performance boost.