added post commenting feature and minor stying

[roman-vasi1enko]

The post pages now have a feature to add new comments by any user and display them (including author name, date, and comment text).

Completed steps:

1. monoose Comment schema
2. new comments.js route to handle createComment requests
3. updated posts.js controllers to include comments data from DB as a res to GET req on page load
4. updated post.js EJS template to display the form and all existing comments on a requested post
5. applied Bootstrap CSS to style the post.js template