Added Comment functionality

[Kirk-Esterline]

Added Comment Functionality to B-U-B

Here are the updated files

1. Posts.ejs

• Added an input field as well as a submit button
• Additionally, a loop to iterate over comments tied to each post

2. Models

• Added a new Mongoose Schema to collect the various comments made for each post
  (Attempted to add comments to the existing posts schema, however that resulted in each comment over writing the previous comment)

3. Controllers

• Updated the post controller to add two functions and connect additional models
• The comments and user models are now required
• First is an addComment controller that creates a new comment in the database using the comments schema
• The second is an update to the get postPost controller to find all the documents related to that post.

4. If I had more time ...

• A feature that is not included with this PR is adding the userName of the user who commented. Adding the the user _id of the user proved to be straightforward however cross-referencing the user from the comment with the userName proved to take more time than available.