Enable privacy mode by default

[bdresser]

Privacy mode should be "ON" by default, with the option to disable.

This requires

• UI to notify users this change was made
• addition of "force log in" button described in Allow manual enable for legacy sites. #6320, Add way to bypass privacy mode on a per-site basis #6110
• some affordance to help users on legacy sites know that they must force log-in
  • research if we can detect sites trying to read .accounts[0] before requesting access
  • if yes, we can show a conditional UI helper just when necessary
  • if no, we should show something persistent for a period of time
• message from Twitter and Medium

[bdresser]

cc @bitpshr @whymarrh @JSON-LEE13

[omnat]

Note:
conditional UI helper can be a notification / pop-up that leads user directly to the force log-in option. cc @cjeria

[whymarrh]

• some affordance to help users on legacy sites know that they must force log-in
  • research if we can detect sites trying to read .accounts[0] before requesting access

Refs #6384 (comment) and #6384 (comment)

The answer is yes, we can detect (pending some implementation details) sites that aren't using the 1102 flow.

[bdresser]

design issue: MetaMask/Design#54