Improve thumbnail rendering

When an image is added to a page as a thumbnail, it renders unclear. Inspecting the storage format shows that the image is stored as a thumbnail:

<ac:image ac:thumbnail="true" ac:height="250"><ri:attachment ri:filename="AddEmailAccount.PNG" /></ac:image>

However, adding a border to the image sharpens it, and the storage format shows that it's no longer rendered as a thumbnail:

<ac:image ac:height="250" ac:width="141"><ri:attachment ri:filename="AddEmailAccount.PNG" /></ac:image>

For a consistent user experience, we should either fix the thumbnail rendering, or remove the "thumbnail=true" flag altogether.

Steps to reproduce

1. Add an image to a page
2. Select one of the preset sizes, or add a custom size
3. Save the page and view it
4. Edit the page
5. Add a border to the image
6. Save the page and view it

Expected result

The thumbnail image will be sharp in both situations

Actual result

The thumbnail is rendered unclear when added without a border. After adding the border, it renders with higher clarity.