Changing B2C implementation to verify policy format

[abhidnya13]

• Changing implementation to check for policy string format instead of b2clogin.com
• This supports custom domains with validateAuthority = True and sovereign clouds

[rayluo]

Thanks for all your effort! This PR looks very clean!

PS: I noticed that all conversation this PR happened off-line. So documenting some reasoning of our implementation for future auditing/reference.

• tweak it so that we do not have to do the split(...) call twice.
• Besides, the latest implementation would wrap the long line by using implicit line continuation
• and skip an unnecessary ternary operator.