A mistake in the sqlite porting led to "Please select an asset"
disappearing. This centralizes that code in Helper, and uses the code
in Licensescontroller and componentscontroller.
Also use the proper name on the components checkout page.
AssetMaintencesController reuses the same underlying code as
licensescontroller, but we don't want "Please select an asset" to be an
option there, so I'm not changing that code.
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.