post yaml apps in the gui

remove the old tab structure for "add-app" and supply one with just the bits that work,
including a new YAML tab at the top level

simply paste valid yaml into this tab to test, for example any of the yaml files listed here:

https://github.com/brooklyncentral/brooklyn/tree/master/usage/launcher/src/test/resources

(some of those may need updating, i've been using them the past few weeks, but the `java-web-app-and-db` one is confirmed working as of today; as a separate activity we absolutely need a strategy for sharing/maintaining yaml files!)