Improve the help dialog UX

Improve the help dialog UX

- The search field is no longer autofocused. This is to allow the following
  points. It also allows to try out commands while the dialog is open; see #619.
- The scrolling commands now scroll the help dialog if it is open (regardless of
  whether it is scrollable or not, instead of confusingly scrolling the page
  behind it).
- The search field is focused by pressing `/` (or `a/` or `g/`).
- The search field is hidden until focused, reducing the risk of it obscuring
  text.

Fixes #643.