Add support for aural-only and visual-only elements

Summary:
Ref T4843. This adds support to javelin_tag() for an aural attribute. When specified, true values mean "this content is aural-only", while false values mean "this content is not aural".

• I've attempted to find the best modern approaches for marking this content, but the aural attribute should let us change the mechanism later.
• Make the "beta" markers on application navigation visual only (see T4843). This information is of very low importance, the application navigation is accessed frequently, and the information is available on the application list.
• Partially convert the main navigation. This is mostly to test things, since I want to get more concrete feedback about approaches here.
• Add a ?__aural__=1 attribute, which renders the page with aural-only elements visible and visual-only elements colored.

Test Plan:

Reviewers: btrahan, scp, chad

Reviewed By: chad

Subscribers: aklapper, qgil, epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8830