Onboarding Flow if user first clicks from any non-web or unsupported page

[ianb]

Now it goes to My Shots, but that doesn't make much sense.

[johngruen]

@ianb changed the name of this issue to be more expansive

Current

If a user clicks the Screenshots icon from any prohibited page (about:x, frameset etc). We either show an error notification or redirect them to "my Shots"

Expected

The user should be onboarded

Proposal

Maybe we should pop a new tab on the screenshots home page and onboard users on top of that page, that seems like the easiest thing to do.

[ianb]

2e5ce26 sends the user to https://screenshots.firefox.com/#hello and starts the screenshotting process (which also invokes onboarding) when the user tries to enable Screenshots on an unshootable page