Recently, GitHub changed these so that they only appear on hover, which is nasty for screen reader users.
Fixes jcsteh#30.

- Makes the filenames in diff viewers level 2 headings.
- Moves the filename first, rather than after a bunch of controls for that file.
- Makes the file contents a region labelled by the filename. Useful when navigating by table rather than by heading.

…n, set role, set label).

1. Pressing alt+1 focuses the first chat in the Chats list.
2. Pressing alt+2 focuses the last message in the active chat.
3. Pressing the context menu key opens the context menu for the current chat or message.
4. With NVDA, the reaction picker no longer switches to browse mode and starts reading all the reactions.

1. Pressing alt+1 focuses the first chat in the Chats list.
2. Pressing alt+2 focuses the last message in the active chat.
3. Pressing alt+3 focuses the message composer text box.

… switching away from the browser tab.

…wn contacts.

…s and non-English text.

Facebook Messenger is also accessible at facebook.com/messages, which
uses the same UI and ARIA roles as messenger.com. This adds an @include
so the accessibility fixes and keyboard shortcuts (Alt+1/2/3) also work
on that URL.