refactor: change deadline to endTime
feat(Task): add start times to tasks

Add startTime variable, getter and setter
Add more constructors so constructor no longer needs to be called with
null parameters
Add tests

BREAKING CHANGE: startTime variable added to Magic8Task constructor.

Constructors:
id, desc, startTime, endTime, tags
desc, startTime, endTime, tags
desc, startTime, endTime
desc, endTime, tags
desc, endTime
desc, tags
desc
task