Show Introduction and Responses in UI

Currently, the introduction client shows the introductions and the responses to them only in system notifications. See the related wiki page for how it is done at the moment.

If the user signs out of Briar before responding via the notification, the notification will be removed leaving the user with no way to respond and the invitation protocol in limbo forever. Therefore, the received introductions should be shown somewhere else in the UI as long as they are not responded to.

Very similar to that are invitations to participate in a forum where there's also a message the user needs to respond to and that needs to be visually presented to the user in some way. That functionality is tracked in #121 (closed).

Other related tickets are Show new messages/forum posts on dashboard and Show recent activity on the dashboard which focus on the Dashboard which does not exist anymore. It has been proposed to show introductions/invitations in the navigation drawer in the appropriate section, so either Contacts or Forums. There, the number of unread messages could also be shown. Later other notifications might be needed e.g. for Blogs or RSS feed reader. So a unified UI concept would be welcome.

Subtask of #118 (closed).