reopen shield-study-2 on release.

[gregglind]

• remove 2 prompts
• go to the new url:
  • https://addons.mozilla.org/en-US/firefox/shield-study-2?utm_source=a
  • https://addons.mozilla.org/en-US/firefox/shield-study-2?utm_source=b
• release / en-us only.

[ghost]

r+ on visual review. Ping me if you want proper testing.