add comment functionality (+ like/delete)

[nicoledicochea]

• Created a comment controller and model.
• CommentSchema contains the following properties: comment, likes, post, user, userId, createdAt.
• Added a form in post.ejs to submit a comment.
• Comments are displayed along with the commenter's username.
• Added functionality for liking comments.
• Conditional in post.ejs determines that if the logged-in user matches the user that made the comment, the trash can icon will appear and the comment can be deleted.