Ability to update app name

[benjreinhart]

This also:

• Introduces app context because that object is A) going to be used in many places and B) we now have some functions to encapsulate that update the object.
• Moves the app header react component out of "editor" because it's not a header just for the editor.

[nichochar]

Would prefer one of those "click and it becomes and input which saves on blur", but this is def good first pass.

Nice on the app context