startDate and endDate should be optional while creating timeline/milestone

[vikasrohit]

As of now setting startDate and endDate are mandatory while creating a timeline or milestone. This makes timeline dates inconsistent with the parent phase/product because we are not setting startDate of a phase when it is created. So, we should allow skipping dates field at the time of creation and set the status of the milestones to be draft.

fyi @RishiRajSahu

[vikasrohit]

As discusses @RishiRajSahu, not implementing it for now as we are treating timeline module independent and don't want to change its implementation based on project module unless it is logical change to have in timeline or we don't have any alternative in calling module (which is project right now).
For handling null startDate on phase, we can simply don't read values from the timeline and milestone objects when rendering timeline for that phase. And when user updates the start date of a phase, we can update the dates of the timeline and milestones.