Enable user reaction to forum invitation

Also updates Briar Core to include briar!1743 (merged)

Can be most easily tested right now by not accepting forum invitations automatically in our TestUtils:

diff --git a/briar-desktop/src/test/kotlin/org/briarproject/briar/desktop/TestUtils.kt b/briar-desktop/src/test/kotlin/org/briarproject/briar/desktop/TestUtils.kt
index e06f6cd..f4dfcf2 100644
--- a/briar-desktop/src/test/kotlin/org/briarproject/briar/desktop/TestUtils.kt
+++ b/briar-desktop/src/test/kotlin/org/briarproject/briar/desktop/TestUtils.kt
@@ -169,13 +169,13 @@ object TestUtils {
         }
 
         // accept invitation at all contacts
-        drop(1).forEach { app ->
+        /*drop(1).forEach { app ->
             app.getEventBus().addListener { e ->
                 if (e is ForumInvitationRequestReceivedEvent) {
                     val contact = app.getContactManager().getContact(e.contactId)
                     app.getForumSharingManager().respondToInvitation(forum, contact, true)
                 }
             }
-        }
+        }*/
     }
 }

Closes #411 (closed)