Support Vi like keybindings for navigation as an option

Many web clients like Gmail are using Vi keybindings which are very popular within a significant group of software developers. I recommend adopting this best practice.

The issue does not identify any files, tests, or entry points. Start by locating the web client's navigation and keyboard-input handling, then define the supported Vi keybindings and an option for enabling them; completion would require tests covering the selected bindings and option behavior.

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