i18n: Translate the interfaces!

A very big thanks to @thane-de-loth for his proof of concept and many others for getting the effort started. It took a long while to get things rolling on our end.

This pull requests makes the client, agent, and admin panels translatable and imports translations for the redactor editor and the jQuery date picker widgets.

Crowdin will be used to translate the strings. I will be posting the strings and creating an online site for in-browser translations soon. If anyone wants to get started in the mean time, a PO file can be generated using:
```sh
php setup/cli/manage.php i18n make-pot > /path/to/output/messages.po
```
Install the current language pack from osticket.com/download-edge and compile the special `mo.php` file using
```sh
php include/class.translation.php /path/to/compiled/messages.mo
```
Place your `mo.php` file into `include/i18n/<lang>/LC_MESSAGES`

This will be done automatically with the language pack building eventually.