Refactor shortcuts code

[Timur Ismagilov]

There is that file static/shortcuts.js that handles the shortcut and the shortcut cheatsheet logic. It is actually hard to understand, at least for me. I want there to be a refactoring. Myself, I tried to do it a couple of times, but always ended up getting lost and dropping it, like today.

So, I want the following:

• Make the code shorter. Currently, there are 370 lines.
• Make the code more declarative.
• Do not use that <template>, remove it. If you need the template, just construct it in-script. It makes more sense.

Extra goals:

• Make it extensible. Provide some global function (window.rrhAddShortcut, maybe) for customization. Currently, I have no idea how to add my own shortcut on my wiki. Maybe it is possible.
• Localize it. Currently, only the close button's aria-label is localized.