User and Post ID in each other's schema

[singohuen]

• As per the requirement, a post should only have maximum one user. The user field in Post schema shouldn't be an array.
• I can tell from data modelling that you are planning to store User and Post in two models/collections. But when a post gets created, you only save userId in Post, without storing postId back to User.
• The userId in Post collection is stored as String instead of ObjectId.

Please make a branch to fix whatever mentioned in this issue.

[1StephenLi1]

Fixed & pushed to branch fixed#1.