Support creating taxonomy documents from within the content perspective

Currently, creating a new taxonomy document can only be done from within Essentials. We should make it possible to perform this task from within the content perspective.

1. Add a "New taxonomy" context menu item when in the Taxonomy section of the content perspective.
2. Either:
   • Prompt only for the document name, and extend the taxonomy editor view to allow editing the supported locales for that document.
   • Prompt for both the document name and the set of supported locales before creating the document.

The first option is preferred, because it provides more flexibility to the customer to change locales for the taxonomy after it is created.