Show group invitations and responses in private conversation

briar-android/res/values/strings.xml

...org/briarproject/android/contact/ContactListFragment.java

...rg/briarproject/android/contact/ConversationActivity.java

...rc/org/briarproject/android/contact/ConversationItem.java

...briarproject/android/contact/ConversationRequestItem.java

briar-api/src/org/briarproject/api/clients/BaseMessage.java

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

...ject/api/privategroup/invitation/GroupInvitationItem.java

...t/api/privategroup/invitation/GroupInvitationRequest.java

.../api/privategroup/invitation/GroupInvitationResponse.java

...-api/src/org/briarproject/api/sharing/InvitationItem.java

...c/org/briarproject/api/sharing/SharingInvitationItem.java