Refactoring settings.py

[stlim0730]

As the most important file in this Django project, settings.py must be in better shape in four senses.

1. Scrub sensitive information from the file. Only the template is shared on the remote repository. Each contributor fill out the template on the local workspace.
2. Allow flexible settings on database, web server, etc. It's currently hard-coded for Heroku-specific settings.
3. Remove duplicate declarations and assignments. It has plenty of them now.
4. Include missing details such as admin information. It's not crucial to run the application, but it should have as much information as possible.

This issue is also a starting point of #37. I'm assigning myself.

[stlim0730]

I came to conclude that this project requires rather app-level refactoring. Closing the issue.