Resolve "Add image support for presentation slides."

Summary

Now it is possible to add an image in a jpeg and png format to a custom slide.

Editing interface:

The presentation with the image:

Images are cutoff if the size does not fit:

Checklist

All items on this checklist should be checked.

• I have added a changelog entry.
• I have added tests for the functionality that I added.
• I adhere to the style guide.

How to review

• Go to a session and edit the presentation
• Add a custom slide and an image in it
• Then present this slide and see if you can see an image

Known Bug

The adding and showing of the image works fine (as far as I am aware), but there is a bug when you go back to the editing page after adding an image. Since this MR was getting big, I allocated that to the new issue #771.

Issues and tasks closed

Closes #739