Updates to locales, confirmation email, invoice, and admin menu

[davekiss]

A few notes for these updates:

• The translations in this extension are out of sync with Spree's best practice according to conventions used in https://github.com/spree/spree/commit/d4ff64bf865dfddf900d04ec8b9acc163d1e6838 - I've gone through and updated the I18n to reflect those changes.
• The admin product tab didn't look like the remaining other tabs (missing icon) - I've chosen an icon and added it to the admin product tab override.
• The order confirmation email was out of date and was using hardcoded strings instead of the Spree I18n model. I've updated the email accordingly.
• The invoice only shows the extension of the file available for download. We sell .zip in our store, and instead of showing only the extension, I propose showing the entire filename, so that the user knows exactly what they are getting when they click on the link.

Open to comments, suggestions, or edits!

[JDutil]

Thanks @davekiss I've added to master w/ e4bc344c0f92ddf8088917b21c3ab0018473e06f and 2-2-stable