Adds cancel link to header when not 2FA'd

[el-mapache]

Why:
Users might like a way to cancel out of their authentication process

[el-mapache]

• We still see the cancel link on the Account Locked screen...not sure if this is desirable
• Clicking cancel of course gives us the message 'you have been signed out'. Technically this is still true as the user's session is destroyed, but should there be a different message displayed to the user? Something like 'Sign in request cancelled.'

[pkarman]

code and tests look great. I'm fine with language as is but that's not my call.