Register start and end time

[InzeNL]

Partial implemention for #3 This will not close this issue yet.

This pull-request adds the following functionality:

• Recording start and end time
• Fixed-time reservation. This means, when someone cancels the reservation before you, your reservation doesn't move up
• Automatically inserts the reservation in the first available timeslot

This pull-request does not yet add the ability to select a specific start-time. This will be implemented separately, likely after the refactor for #9