Add placeholder when the user has no upcoming events

Please test on both clean and boost. Note that there are still some styling issues with the clean theme. Please report any problems in MDL-58235 . This issue is mainly concerned with making sure the placeholder images are being displayed. Make sure you have no events on your site Create 2 courses that are available now (i.e. start date now or in the past) Create two users and enrol one of them in the course Create an assignment with a due date in one of the courses Log in as the student that is enrolled View the timeline -> sort by courses page Confirm that the course with an event is showing the event Confirm that the course without events is showing the placeholder image for no events View the courses tab Confirm that the courses are showing in the "in progress" section (assuming they are in progress) View the past and future sections and confirm that the placeholder images are showing Log in as the other user View the timeline -> sort by dates section Confirm the placeholder for no events is being shown View the timeline -> sort by courses section Confirm the placeholder image is being shown View the courses tab Confirm the placeholder image is being shown without the past, in progress, and future buttons (since this use isn't enrolled in any courses)