Set avatar for the "course starts in X" message

The message, that for example now shows on https://learn.totem-project.org/courses/course-v1:IWMF+IWMF_KP_FR+001/course/, has a picture left of it, of the person writing the message. According to [this topic](https://discuss.openedx.org/t/how-do-you-deactivate-course-tools/2118/3 we should be able to overwrite things in the course_experience module by editing personal_theme/lms/templates/course_experience/. The avatar gets loaded from https://learn.totem-project.org/static/course_experience/images/home_message_author.a48ad5205f6b.png so I hope we can overwrite it by putting an image in our theme in theme/lms/static/course_experience/images/home_message_author.png.

Here's a screenshot of the message: