Don't put echoSettings in build.sbt anymore and other echo installer fixes

 * make the "have we added this" flags past-tense not -ing
 * we set addingEchoFile when we added the background file,
   and addingBackgroundFile when we added the echo file, fix.
 * we used editedBuildFile for two purposes, change it to
   addedForkInRun and drop the code that added echoSettings
 * fix the filenames in the echoInstaller.html modal to reflect
   current actual filenames
 * add TODO that using applicationReady to compute echoReady
   is broken
Merge pull request #744 from typesafehub/wip/havocp-fix-echo-settings

Don't put echoSettings in build.sbt anymore and other echo installer fix...