Add a feedback alert and a boolean settings flag to turn it off/on.

This adds a boolean flag in settings.yml (default `false`) to show the `i18n`ed feedback alert.  We can change the message anytime we like and then turning the message on/off will simply require changing the `config/settings/production.yml` (instead of pushing releases).

If you want to test this out set `Settings.FEEDBACK_ALERT = <%= true %>` in your settings.yml
 
![feedback](https://cloud.githubusercontent.com/assets/96776/5498119/ecac3e74-86ce-11e4-8647-f1acd1d8f9c2.png)