Disable local learning by default

[ghostwords]

Fixes #2548.

• Disabled learning by default
• Hid hasn't-decided-yet-to-block domains in the popup when learning is disabled, as deciding to block depends on learning
• Removed the option to show no-tracking-detected domains, unless learning is enabled.
• Updated badge and popup to count (cookie)blocked domains only (instead of counting (cookie)blocked + hasn't-decided-yet-to-block domains)
• Updated popup summary text to say "blocked X trackers" instead of "detected X trackers", as "detected" is inaccurate and misleading, at least when learning is disabled. (Consider opening the popup on a website with trackers unknown to Privacy Badger and learning disabled.)
• Similarly, updated the wording and what gets counted in the "tell your friends" message
• Added a prompt to popup (on update for existing users) to visit the relevant blog post
• Updated the new user welcome page
• Added blocking of all widget domains on extension install/update to ensure widget replacement happens for all users by default. (Widgets only get replaced when blocked; now that Badgers don't learn by default, if a widget doesn't get blocked during pre-training for whatever reason, it won't get replaced unless we auto-block all widget domains.)

To be followed by auto-replacing the tracker database on extension update when learning is disabled. Longer term, to be followed by #1299.

The new/rearranged settings on the options page:

The new summary text in the popup:

The prompt to learn more:

The rewritten section in the new user welcome page:

[ghostwords]

@Gowee Have you seen our explanation for this PR? Let me know if you have any questions.

[Gowee]

@ghostwords Thanks for the notifying.
I have read that days ago and fully understand the concerns on fingerprinting and possible attack surface illustrated in the article. I am just a little worried about the coverage of the pre-trained domain set hence "thousands of" sounds like just a tiny fraction of the Internet. The problem appears to be obvious for some websites with less popularity. There might also be other systematic problems. For example, I suppose a few popular websites in mainland China are not visited by badger-sett because their poor (WA)firewall prevents/gives a CAPTCHA for requests originating from foreign countries.
Anyway, I think it is the right direction. Hope the Badger keeps working well in the future.

[ghostwords]

Agreed! Badger Sett's coverage is indeed limited in a number of ways. In an ideal world, we would have rolled out community learning (#1299) before turning off local learning by default. We do not believe Badger Sett alone is enough.