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 #1288

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

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.

Show a proper error message when database is too new or too old.