Text and image not wrapping or resizing in calendar event description

As reported by miralt in MDLQA-13451:

In my opinion long text's and big images don't shown correctly. Image should be resized to size of pop-up window. Text should be wrapping. Please check it. The problem occurred in earlier versions, e.g. 3.6.

I use: windows 7 64 bit, Google Chrome version 73.0.3683.103. Confirm error in firefox 65.0.2 (64). Problem occured in both themes

Steps to reproduce:

1. Log in as an ordinary user and go to the calendar.
2. Click the 'New event' button, add an event title, then click the 'Show more...' link and add a description, including an image or media file and text.
3. Click the Save button, then click on the event.