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Now an invitiation was answered when it is no longer available

This was done, so private responses don't need to include a Nameable already.
Retreiving a nameable is tricky and requires a data migration,
so we just don't do it now.

This removes the public method for retrieving messages
from individual conversation clients
and just leaves methods that require a transaction
to be used by the ConversationManager only.

This also adds `Shareable`s to invitation response
which is a precondition for #561

* Create at least one contact

These don't affect client visibility.

It is possible that a remote DECLINE message arrives short before the
user responds to the introduction.
This will cause a ProtocolStateException which (for now) is just caught
and a generic (existing) error message will be shown.

A fake ACTIVATE message would cause us to activate the transport keys
before the contact has received our auth message,
which would compromise forward secrecy.

When the user attempts an introduction, instead of the introduction
message input field, an explanatory text will be shown and the
introduction can not be made until the last one has been finished.

Closes #308