Implement first prototype of private group message threads

briar-android/AndroidManifest.xml

briar-android/res/drawable/ic_group_white.xml

briar-android/res/menu/forum_actions.xml

briar-android/res/menu/group_actions.xml

briar-android/res/values/strings.xml

...droid/src/org/briarproject/android/ActivityComponent.java

...-android/src/org/briarproject/android/ActivityModule.java

...ndroid/src/org/briarproject/android/AndroidComponent.java

...c/org/briarproject/android/forum/ForumControllerImpl.java

...ject/android/privategroup/conversation/GroupActivity.java

...ct/android/privategroup/conversation/GroupController.java

...ndroid/privategroup/conversation/GroupControllerImpl.java

...ndroid/privategroup/conversation/GroupMessageAdapter.java

...oid/privategroup/conversation/GroupMessageViewHolder.java

...arproject/android/privategroup/list/GroupListAdapter.java

...iarproject/android/privategroup/list/GroupViewHolder.java

...org/briarproject/android/threaded/ThreadListActivity.java

briar-api/src/org/briarproject/api/forum/ForumPost.java

...i/src/org/briarproject/api/privategroup/GroupMessage.java