Factor out generic sharing code from ForumSharingManger

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

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

...st/java/org/briarproject/ForumSharingIntegrationTest.java

...rg/briarproject/ForumSharingIntegrationTestComponent.java

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

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

...g/briarproject/android/forum/AvailableForumsActivity.java

...g/briarproject/android/forum/ContactSelectorFragment.java

...src/org/briarproject/android/forum/ForumListFragment.java

...briarproject/android/forum/ShareForumMessageFragment.java

.../briarproject/api/event/ForumInvitationReceivedEvent.java

...oject/api/event/ForumInvitationResponseReceivedEvent.java

...c/org/briarproject/api/event/InvitationReceivedEvent.java

...iarproject/api/event/InvitationResponseReceivedEvent.java

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

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

...rc/org/briarproject/api/forum/ForumInvitationMessage.java

...i/src/org/briarproject/api/forum/ForumSharingManager.java

...i/src/org/briarproject/api/forum/ForumSharingMessage.java

...i/src/org/briarproject/api/sharing/InvitationFactory.java