#47 - multiple UX improvements

[krzysztofrewak]

This pull request is related to #47 and should close #58.

Changes includes:

• remove possibility of having more than one active semester (why it was implemented in the first place?)
• change the way of calculating if course is in active semester (current solution was really really bad)
• reorder items in tables in dashboard
• remove icons from tables
• merge show and edit view of course groups
• adjust all public views to look the same
• add some nicer data seeder

[EwelinaSkrzypacz]

1. On page "Studenci" and "Sekcje" buttons in table are still icons
2. Can we add some space here?