Don't use ClientId.toString() for signature labels.

...main/java/org/briarproject/bramble/api/sync/ClientId.java

...java/org/briarproject/briar/api/blog/BlogPostFactory.java

...va/org/briarproject/briar/api/forum/ForumPostFactory.java

...arproject/briar/api/privategroup/GroupMessageFactory.java

...r/api/privategroup/invitation/GroupInvitationFactory.java

...rg/briarproject/briar/introduction/IntroduceeManager.java