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

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- The order confirmation email was out of date and was using hardcoded strings instead of the Spree I18n model.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!