Refactor main

Small refactor.

• Rename JSONImportForm to JSONScheduleImportForm, as we want to add more json import forms
• In is_event_covered check all blocks if they cover the event, not just the first
• Add more docstrings and comments
• Move merge_blocks out of Event class as it is independent of the event
• Use round instead of int for duration calculation
• If no Default Slots exist, use uniform_time_slots for discretization