Implement toggle bold and italic shortcuts

[kmdupr33]

Fixes #861

This pr pairs with another pr that contains changes to the gutenberg repo. It follows the proposal I laid out here.

This only contains work for the italics and bold shortcuts because the undo and redo shortcuts are already working.

• The main listeners for shortcuts are in the GutenbergViewController and MainActivity that host the React native app. On iOS, this enables us to leverage some of the native shortcut discoverability features that are on iOS as well as making shortcuts "global" for free. If we agree this is a reasonable choice on iOS, I'll need to add localized strings for shortcut discoverability (see checkbox below). For consistency and for making it easy to make shortcuts global, I also implemented the shortcut listener in the MainActivity on android.
• On iOS, since the GutenbergViewController is already pretty long, I've created a ShortcutsCoordinator in the same spirit as the Media*Coordinators. In order for the shortcuts coordinator to do non-trivial work in handling shortcuts, I needed to modifier the responder chain so that it has a chance to handle shortcut events. On Android, the ideal home for this functionality was less obvious, and since the MainActivity is pretty lean, I threw some of the shortcut code there for now. I'm totally open to feedback if anyone has any ideas where that code should live.
• A key assumption I made here is that keyboard shortcuts for us are always global and don't have target elements, which is unlike web's implementation. I figured this was kosher, but if I'm wrong about this, I'll need to rethink some things on the js side and on the native side.

To test:

If you want to test the bold shortcut, its as simple as clicking a textview and pressing modifier-b.

If you want to test italics, you'll have to run the apps in release mode so that the developer menu is disabled and the toggle inspector shortcut doesn't interfere with the toggle italics shortcut.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[hypest]

Closing this as not updated in a while.