"Don't Bootstrap" Startup Mode

Tor Browser needs a better user experience at start-up, and automatically bootstrapping and doing "Smart Things" in the background is the next stage in Tor Browser's evolution. But users should be able to launch Tor Browser and tell it not to bootstrap (before it starts bootstrapping).

Roger and I had some ideas including:

• Add a context menu option for launching but not bootstrap
• Add a key combination (like pressing spacebar) and Tor Browser what that means
• Tor Browser always waits 5 seconds before bootstrapping, and provides a "cancel" button

None of these are really good options, but maybe use this as a starting point and find a better option.

At the root of this is trying to solve three problems:

1. Tor Browser usability depends on tor bootstrapping seamlessly and hiding this detail from users
2. The current configuration may be dangerous after moving into a new environment
3. Tor Browser should have a start-up mode where Tor Browser doesn't send a single packet on the network before the user has the option of configuring tor

(Most likely) 99.9% of users only need (1), but we shouldn't ignore (2) and (3).