Unify all private message requests in one PrivateRequest class

...iarproject/briar/android/contact/ContactListFragment.java

...arproject/briar/android/contact/ConversationActivity.java

.../briarproject/briar/android/contact/ConversationItem.java

...rg/briarproject/briar/api/blog/BlogInvitationRequest.java

...ar/api/blog/event/BlogInvitationRequestReceivedEvent.java

.../briarproject/briar/api/forum/ForumInvitationRequest.java

.../api/forum/event/ForumInvitationRequestReceivedEvent.java

...org/briarproject/briar/api/introduction/Introduction.java

...arproject/briar/api/introduction/IntroductionManager.java

...arproject/briar/api/introduction/IntroductionMessage.java

...arproject/briar/api/introduction/IntroductionRequest.java

...rproject/briar/api/introduction/IntroductionResponse.java

...n/java/org/briarproject/briar/api/messaging/Nameable.java

.../org/briarproject/briar/api/messaging/PrivateRequest.java

...r/api/privategroup/invitation/GroupInvitationManager.java

...r/api/privategroup/invitation/GroupInvitationRequest.java

...org/briarproject/briar/api/sharing/InvitationMessage.java

...rg/briarproject/briar/api/sharing/InvitationResponse.java

...in/java/org/briarproject/briar/api/sharing/Shareable.java

...va/org/briarproject/briar/api/sharing/SharingManager.java