* A11Y: Update bulk selection keyboard shortcuts
Still a draft, but in current state this:
- adds `shift+b` as a keyboard shortcut to toggle bulk select
- adds `shift+d` as a keyboard shortcut to dismiss selected topic(s) (this
replaces `x r` and `x t` shortcuts)
- adds `x` as a keyboard shortcut to toggle selection (while in bulk select mode)
- fixes a bug with the `shift+a` shortcut, which was not working properly
Note that there is a breaking change here. Previously we had:
- `x r` to dismiss new topics
- `x t` to dismiss unread topics
However, this meant that we couldn't use `x` for selection, because the
itsatrap library does not allow the same character to be used both as a
single character shortcut and as the start of a sequence. The proposed
solution here is more consistent with other apps (Gmail, Github) that use
`x` to toggle selection.
Also, we never show both "Dismiss New" and "Dismiss Unread" in the same
screen, hence it makes sense to consolidate both actions under `shift+d`.
* Address review
