Add vi-like keybindings UI option.

### Your use case

#### What would you like to do?

I would like to be able to use vi-like keybindings in the Element.io web UI.

#### Why would you like to do it?

The biggest hindrance to my use of Matrix at this time is lack of a client that offers user experience that suits my preferences. The closest option is WeeChat with the Matrix plugin, but it does not support needed functionality for my needs, that plugin does not work on some platforms, and there is evidently only one plugin for vi-like keybindings (which happens to not be a great design for a vi-like keybinding option).

In short, I would like vi-like keybindings so I will be motivated to use Matrix rather than sticking with *only* irssi and IRC.

#### How would you like to achieve it?

I would like to have a simple, straightforward, easily discovered way to choose vi-like keybindings as an option in the Element.io web UI and, preferably, more UI keybinding configurability in general. Ideally, this would also be incorporated into the Element "desktop" application, so that if and when it gets ported to my preferred platform(s) I will be able to use that as easily as the web application.

### Have you considered any alternatives?

The alternatives at the moment seem to be:

1. WeeChat with Matrix plugin
2. always using IRC instead of Matrix

As indicated in the use case information, the WeeChat option is deeply flawed. Just using IRC instead of Matrix defeats the interest in Matrix privacy and pseudo-decentralization capabilities.

### Additional context

I manage some communities in IRC that may benefit from a move to Matrix, but are not really migratable to Matrix at this time due to hindrances for my own preferences in chat client usage, as well as due to client portability limitations so that not all user platforms for these communities are reasonably easy to get set up with acceptable Matrix clients. I am a privacy advocate, and others in these communities have similar sympathies, but moving to Matrix under current conditions would result in community fragmentation.