Also change the logo to a logout icon

This is done via another preference in the settings screen
and an action button attached to the notification itself

Fixes #888.

This is just a first rough implementation.
A real UI designer should look over this.

Closes #1096

Closes #1288

Closes #1297

* Force LTR by prefixing language names with the LRM marker
* Add Polish
* Cleanup

* Create at least one contact

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.