Updated akka-bbb-apps project to use sbt 1.2.6 and scala 2.12.7

akka-bbb-apps/build.sbt

akka-bbb-apps/project/Build.scala

akka-bbb-apps/project/Dependencies.scala

akka-bbb-apps/project/build.properties

akka-bbb-apps/project/plugins.sbt

...igbluebutton/core/apps/users/RegisterUserReqMsgHdlr.scala

.../test/scala/org/bigbluebutton/core/AppsTestFixtures.scala

...luebutton/core/domain/MeetingInactivityTrackerTests.scala

...scala/org/bigbluebutton/core/models/GroupsChatTests.scala

...core/pubsub/sender/ReceivedJsonMsgHandlerTraitTests.scala

.../scala/org/bigbluebutton/core2/testdata/TestDataGen.scala