Remove formtastic, carrier_wave & spreadsheet gems

In the process, a lot of redundant partials also get removed
Modified code for file uploading - not using FormData()

Got rid of carrierwave. And therefore doing sanity checks on mime-types internally
Doing away with carrierwave and therefore ability to upload student list as .xls

Teachers now create sektions and students enter the supplied sektion code
to enroll in one