[Feature] Add account selection interrupt for signed in users

[pauliusuza]

Feature Request: Add an account selection interrupt within the authorization flow for signed in users

Use case: A web app allows its users to authorize multiple steem accounts

Current behavior: When user is redirected to https://steemconnect.com/oauth2/authorize..., if he/she is already logged the authentication flow silently proceeds and redirects back to the url specified in the redirect_uri parameter.

Expected behavior: When user is redirected to https://steemconnect.com/oauth2/authorize..., if he/she is already logged into steemconnect, an interrupt would be displayed:

Clicking on switch account would immediately prompt the user to sign in

[pauliusuza]

WIP https://github.com/steemit/steemconnect/pull/217

[pauliusuza]

Solved by #217