Add "aria-live" announcements to key page/tab transitions.

[sraby]

The Accessibility Audit mentions certain transitions that could benefit from this added element. Once we have our <AriaAnnoucement> component moved out of tenants2 and into our shared monorepo, this should be pretty straightforward to add.

[toolness]

Note that this kind of thing will also be useful for our "subscribe to our newsletter" form too!