Update Web Authentication to reflect changes in SEP-10

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] I have read the CONTRIBUTING document.
[x] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[ ] I have added tests to cover my changes.
[x] All new and existing tests passed.