1) Missing $admin_user in AccessoriesController::postCheckout(). Fixes#2183
2) AssetsController::postCheckin used the hardware url for the user link, not the user url
This allows for centralizing the category fetching code more and fixes
an error in asset model viewing (#2118). Also add a few translated
strings and standardize on a base of 'Select a *' for the default value
in our lists.