fix(client): OAuth/redirect users who paste the verification link into the original tab should be redirected to the RP.

Save a bit to sessionStorage whenever the user signs up or initiates a password reset. When the verification tab opens, it looks for this bit. If the bit is set, that means the user overwrote the signup/reset password tab with the verification tab and the user should redirect to the RP when the OAuth flow completes. If the bit is not set, this is either another tab or another browser, and the user should *not* go back to the RP.

fixes #1816
Merge pull request #1947 from mozilla/issue-1816-redirect-verify-same-tab

fix(client): OAuth/redirect users who paste the verification link into the original tab should be redirected to the RP. r=vladikoff