Comments: Add comments working with database

[melvinlau]

Feature added to add comments. Capybara feature test passing.

Note: Comments can be added on the posts page, which then should redirect back to posts. Although a comment is created in the database, the posts page does not reload itself automatically. On to do list.

Notes to aid resolution of merge conflicts:

• Comment model and migration file were added, i.e. db schema to be updated
• Routes file to include comments as resources
• The posts index view has been updated to add and/or show comments under specific posts