This commit updates the way in which we bind key sequences to functions
when implementing keyboard shortcuts.

Before this commit, developers would directly bind a key sequence to a
handler:

```js
Mousetrap.bind('p b', handler);
```

After this commit, developers will instead define a new "command" in
`~/behaviors/shortcuts/keybindings.js` and bind to the result of
`keysFor(command)`:

```js
import { keysFor, TOGGLE_PERFORMANCE_BAR }
from '~/behaviors/shortcuts/keybindings.js';

Mousetrap.bind(keysFor(TOGGLE_PERFORMANCE_BAR), handler);
```

`keybindings.js` handles returning the appropriate key sequence for
the command (or it returns `[]` if the command is disabled).