Fixes to models, finished courses#new, sketch of topics#new

* Models were fixed: removed unnecessary constraint to validates foreign id's which caused problems with insertions and updates.
* Finished courses#new view and controller. Temporally using a multiple select box to select categories for a course.
* Created sketch of topics#new view and controller
* Created function read_zip: it will read the contents of an uploaded zip to create a topic. Currently, it only passes through all the file entries; definition of zip format needed.