Post title writing flow

[koke]

As part of #182, we added the post title in the apps themselves, which has lead to some issues. For the beta, we'd want:

• Title should have no border
• Title shouldn’t be fixed (it should scroll with the content)
• Title and content should be aware of each other in terms of focus. When the title is selected, the toolbar should only contain global tools (Inserter, Undo/Redo, Hide Keyboard)
• The "hide keyboard" button should work on the title as well
• Pressing enter on the title should move to the next block or split a new one if necessary [https://github.com/Handle pressing Enter.key on title in RN #502]
• Integrate the Noto font and use it on blocks
• Make sure undo / redo works.
• The title should be saved in the global store instead of part of AppContainer.js's state.
• The title placeholder should read "Add a Title". (ref)

[hypest]

Allow me to add that it would be nice if we could timebox (the least) an experiment around this: render the title inside the RN side, effectively introducing the first mobile non-content block.

[diegoreymendez]

All tasks are completed... this issue will be closed once all PRs are merged.