[Branch] Add Student Id field

[sudogene]

Student Id is aimed to be a unique identifier of a student. The end goal is to have Id creation be automated and not part of the user input. Uniqueness of the value is done by the application, freeing user from the burden. This has not been implemented yet as of this mid iteration.

The Id field

• String value, must be numeric
• Padded with leading zeroes ONLY for UI display new Id("1") ==> "001"
• Value itself has not been padded
• CANNOT be edited, EditCommand does not support editing of Id

Current iteration for pre-mid-v1.2

• Requires manual input from user add n/John Doe id/2 ...
• Uniqueness of Id value has not been implemented yet

Additional feature

• Able to find by student id find id/2

[yejiadong]

LGTM.