[WIP] Do not merge: First approach/draft on idrop-web login simplification

Please don't merge this pull request yet, I'm just exploring exploring how hard could it be from a beginner in Groovy/Grails to introduce the changes outlined in https://github.com/DICE-UNC/idrop/issues/28 in idrop-web3.

Question for @michael-conway: how does one disable GSP page caching in development? After commenting the code in the view it keeps appearing in the browser anyway across re-deployments of the war file (`grails run-war`) and browser cache cleanups... which cache am I missing? :/