delay page transition until we have all resources

[pieh]

This does couple of things:

• Gatsby won't navigate to new page until resources for it are loaded (previously path would change after 1s even if resources are not yet loaded, so this would cause scroll jump without page change)
• If navigating to path that doesn't exist, it will wait to load resources for 404 page (to avoid similiar scroll jump as in point above)
• Adds new browser APIs, so loading indicators can be implemented without hooking into Gatsby internal messages

Closes #3372

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[m-allanson]

👍 LGTM!

[KyleAMathews]

Great work @pieh!