[#31] Material: Contributor Subscribe to Comments on His/Her Uploaded Materials

Closes #31 (closed)

Changes:

1. Setup Django mail engine to use Google Mail SMTP by default, along with some configurations
2. Add boolean field is_subscribing_to_material_comments on User model/form + migration
3. Implement notification mail sending to material uploader on new commment, if they subscribed
4. Will not send notification mail when the material uploader comments to their own material.