Create ImageBox.thumbnail when ImageBox.image is being set

[cydanil]

Description

This closes #357 , in which it's described how a recipe cannot be modified, because of a save error when serializing the image and its thumbnail.
The problem is that the thumbnail attribute was only created when loading an image from file, and not when loading the image from bytes (ie. from the database.)
This resolves the issue by moving the thumbnail creation to whenever the image is loaded.

How Has This Been Tested?

This was tested by editing the title of a recipe (Hong Kong ice cream -> Hong-Kong ice cream).
Without the changes, an AttributeError is raised.
With the changes, the application behaves as expected.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.