replaced prodDB/devDB with db with template string

Justification:
- It is helpful to have the mongoose constructor contain a template string for mongo URL.
- The user was expected to copy the same operations for both `prodDB` and `devDB`,
  but they can come to that conclusion themselves after seeing this base case
  template file for just a single connection.
used .TEMPLATE.js postfix on db populate script

Justification:
- same style is used for config.TEMPLATE.js
- user can quickly see what they need to modify to run the project
added MongoDB instructions to the README

Justification:
- Wanted to make the process more obvious to someone wanting to quickly setup