* This fixes a denial-of-service exploit that would allow the client to
generate an infinite number of thumbnails and fill up the storage completely.
Since the client had control over the requested thumbnail sizes, it could make arbitrary requests for thumbnails, and every time the backend did not find an already generated thumbnail with the specified sizes, it would happily generate a new one.
* Remove the ability of the client to decide thumbnail dimensions and
only let the back-end do this by reading the configuration.
* Limit the number of generated thumbnails per file to only one, with
"landscape" dimensions (4/3).
* Use CSS "object-fit" property to adjust displaying of landscape thumbnails into squares. Ref: https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

* We now use only one thumbnail file src for both square and rational
thumbnails so remove superfluous requests.
* Set the same src for both square and landscape dom classes.

Only compute thumbnail configured dimensions on thumbnail API requests.

* Down-sampling should only be requested by the backend.
* For now remove the client-side logic entirely since it is both
superfluous and vulnerable. This can be implemented again on the
back-end if necessary.