Support selecting a user from dropdown

[apobbati]

Currently, there seems to be no easy way to select from a preset of users without actually entering the associated attributes. I'm creating an enhancement to make this work when an array of users is specified in the configuration file (config.js)

[apobbati]

Adding an image for clarity:

[john-aws]

While I think this PR is potentially useful, it seems to have (at least) a couple of problems:

1. it doesn't sync the attribute statements when you select a user from the dropdown
2. if you pick the 'Saml 2' user from the dropdown and click Sign In, the posted assertion that an SP sees is in error and emits "Error: SessionIndex not an attribute of AuthnStatement." because the added users have no SessionIndex