Improve accessibility of some UI elements (forms, icons, timeline)

Summary: Ref T4843. Chips away at a few more things.

Test Plan: Used VoiceOver and got a generally more sensible-seeming result.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8978