[Admin] Created default admin site, customized Comment change page on admin site

[JustinMi]

This PR is pretty short, I created the boilerplate admin site, and also added customization for the Comment change page on the admin site.

• Created admin site
• Added sorting options for Comment change page: by message (alphabetically), by time, by language, by flagged, and by tag
• Changed it so that empty messages will show up as "-- empty response --".
  • In order to do this, I wrote a custom migration that made Comments.messages nullable, and set all empty messages in my MySQL database to be NULL
  • edited the make_comments view function so that future empty messages will also be NULL
  • We will have to make sure the database change is reflected in the prod server