Minor document rendering fixes: dropdown for synchronous files, URI normalization for default renderers

Summary:
Depends on D19258. Ref T13105.

• When the default renderer is an Ajax renderer, don't replace the URI. For example, when viewing a Jupyter notebook, the URI should remain /F123, not instantly change to /view/123/jupyter/.
• Fix an issue where non-ajax renderers could fail to display the dropdown menu properly.

Test Plan:

• Viewed a Jupyter notebook, stayed on the same URI.
• Changed rendering, got different URIs.
• Viewed a JSON file and toggled renderers via dropdown.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19259