Show "Create Notebook Job" context menu item for all file types

[JasonWeill]

Problem

Some environments may be able to run file types other than notebooks (ipynb files) but we only show the "Create Notebook Job" context menu item in the file browser when the user right-clicks on an ipynb file.

Proposed Solution

Let environments declare which file types they can run (see #316). Show the "Create Notebook Job" context menu item in the file browser when the user right-clicks on any file (but not a directory).

In the create form, show only environments that can run the chosen file. If no environments are compatible with this file, the create form should show an error and should not allow the user to create a job or a job definition.