ForumManager: Added parent id to metadata and a unit test

...ests/src/test/java/org/briarproject/ForumManagerTest.java

...test/java/org/briarproject/ForumManagerTestComponent.java

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

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

...r-api/src/org/briarproject/api/forum/ForumPostHeader.java

briar-core/src/org/briarproject/clients/ClientsModule.java

briar-core/src/org/briarproject/forum/ForumManagerImpl.java