Invalidate Pontoon session when logging out from FxA

[mathjazz]

When logging out of the connected session for Pontoon in accounts.firefox.com, the session is actually not invalidated. It would be nice to address this in order to not mislead the users into thinking they are signed out.

Steps To Reproduce:

1. Go to accounts.firefox.com and sign in.
2. Go to pontoon.mozilla.org and sign in with same Firefox Account as used in step 1.
3. Go to accounts.firefox.com and select Settings > Connected Services > Pontoon > Sign out.
4. You get a pop-up stating that you are Logged out of Pontoon.

Expected result:

1. You are actually logged out of Pontoon.

Actual result:

1. You are NOT logged out of Pontoon.

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1847522