Add ability to toggle html/visual modes through the bridge

[pinarol]

This change adds switch to html/visual mode messaging to the bridge and demonstrates it on the example app.
On RN side: Updates the BlockManager to handle TOGGLE_BLOCK_MODE updates and re-render its contents.

Part of #182

To Test

Test 1:

• Run yarn install & yarn start and yarn ios as usual to open the example app
• Tap (...) button on the rightside of the navigation bar
• Tap "Switch to HTML"
• Verify that HTML editor is shown
• Tap (...) button on the rightsize of the navigation bar again
• Tap "Switch to Visual"
• Verify that visual editor is shown

Test 2:

• Reopen the example app
• Tap (...) button on the rightside of the navigation bar
• Tap "Switch to HTML"
• Edit something in html edit mode(do not switch back to the visual mode)
• Tap Save in the navigation bar
• Verify that you see the edited version of the html in Xcode logs

Test 3:

• Reopen the example app
• Tap (...) button on the rightside of the navigation bar
• Tap "Switch to HTML"
• Edit something in html edit mode
• Tap (...) button on the rightsize of the navigation bar again
• Tap "Switch to Visual"
• Verify that you see the edited version of the html in visual mode
• Tap Save in the navigation bar
• Verify that you see the edited version of the html in Xcode logs

Also:

• Verify that both "Inspect blocks" and "Switch to html mode" switches are removed.

[koke]

I pushed a commit that gets rid of the extra padding at the top, which wasn't that noticeable before, but looked weird without the toggles

[koke]

I tested this and works great, iOS code looks good as well.

[etoledom]

Great job @pinarol ! It looks super good and it's working flawlessly 🎉

I got one question as code comments sections, but whatever is the answer I'm happy to ✅ and !

[etoledom]

A little note @hypest .
By merging this PR, we are going to lose the ability to switch to HTML in the Android example app, until the bridge and a native-side switch is implemented there.

[etoledom]

It would be good also, at this point, to bundle a new App.js file, since the current one is quite outdated and WPiOS won't recognize the new bridge message without this code.

But it could be on a follow up PR too. 👍
cc @koke

[koke]

I'll be pushing a new version of the bundle tomorrow with all the latest changes