Refactor Forum Activity and adapters to be re-used for private groups

briar-android/res/layout/activity_forum.xml

...oid/src/org/briarproject/android/forum/ForumActivity.java

...ndroid/src/org/briarproject/android/forum/ForumEntry.java

...rc/org/briarproject/android/forum/NestedForumAdapter.java

...src/org/briarproject/android/forum/NestedForumHolder.java

...t/android/privategroup/conversation/GroupMessageItem.java

...src/org/briarproject/android/threaded/NestedTreeList.java

...oid/src/org/briarproject/android/threaded/ThreadItem.java

.../org/briarproject/android/threaded/ThreadItemAdapter.java

...g/briarproject/android/threaded/ThreadItemViewHolder.java

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

...ava/org/briarproject/android/forum/ForumActivityTest.java

...ava/org/briarproject/android/forum/TestForumActivity.java