Redesign: keyboard shortcuts

[bwindels]

• add shortcuts
• add shortcuts dialog

Looks reasonable if we don't allow the shortcuts to change.

[aaronraimist]

First opened this as a separate issue (#7747) but it probably fits here:

There isn't any good way to determine what they available keyboard shortcuts are. I think we should start adding them to tooltips and maybe add a dedicated interface, something like iOS's keyboard shortcuts viewer.

or like StackOverflow's help sheet:

Related: #7165 #3232

[t3chguy]

There are other issues representing adding various other keyboard shortcuts, I will be repurposing this one just for adding a means of discoverability in the app itself.

[ptman]

This probably makes https://github.com/vector-im/riot-web/blob/develop/docs/shortcuts.md redundant?