Fixes #504. Automatically enable dependent plugins

This is enforced on the server side during setting validation, and
also reflected in the web client. Switches for dependent plugins
get set to the "on" state if they were not already on.
Fix failing test, test new validation behavior
Add a test for plugin dependency enabling
Merge pull request #507 from girder/plugin-auto-select-dependencies

Fixes #504. Automatically enable dependent plugins