Merge branch '708-private-group-remove-new-member-announcement' into 'master'

...c/test/java/org/briarproject/PrivateGroupManagerTest.java

...roid/privategroup/creation/CreateGroupControllerImpl.java

...src/org/briarproject/api/clients/ContactGroupFactory.java

...rg/briarproject/api/privategroup/GroupMessageFactory.java

...pi/src/org/briarproject/api/privategroup/MessageType.java

...rg/briarproject/api/privategroup/PrivateGroupManager.java

...src/org/briarproject/clients/ContactGroupFactoryImpl.java

...rg/briarproject/privategroup/GroupMessageFactoryImpl.java

.../org/briarproject/privategroup/GroupMessageValidator.java

...rg/briarproject/privategroup/PrivateGroupManagerImpl.java

...src/org/briarproject/privategroup/PrivateGroupModule.java