Support password authorization.

[umi0410]

Description: it looks great to support authorizaiton by password on CodePair console.

Why:

1. Someone wants to block public access to their CodePair.
2. Password authorization is a proper authorization method because CodePair is not a complicated but a simple service.

For example, Go Playground lets us share codes in a simple way. Although there is no password, they create a unique link to share on every click on the "share" button. I don't suggest a complex authorization but a minimal security (e.g. password) which is proper to CodePair because it is a simple pair programming service.