[Workbench] Log in into different clusters
As a user I would like to be able to log in into different Arvados instances
so that I can search and browse different Arvados data from my home cluster
- list available Arvados instanced with log in button
- local storage for instance tokens
- table with column: Cluster ID, Username, Email, Status, button "Log in" or "Log out"
- input for manual adding of new cluster