Generate a new Forum Application

This application will host the forums created by the users, and it will
use the subscribem engine and another engine for handling forum
features.
Installs and configures the subscribem engine.

Adds the engine to the gemfile, mounts the engine on the routes
files, and configures the db file.

Once everything is in place, we need to run the following commands:
- bin/rake db:create
- bin/rake railties:install:migrations
- bin/rake db:migrate