R1:e8cebb7da5a4

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…