VID-1854 square thumbs

/CC @lizlux 
Here we're essentially getting the width of the image.  Its going to be used for both width and height, but width is the controlling factor.  So maybe we can call this ```getImageWidth```?  If we had multiple methods for getting the width, adding the ```ByXandY``` would make sense, but here I think it simple is better.

Also, I think we can simplify the this function and remove the other function (getDimensionMultiples) and just do something like:
```php
if ( empty( self::columnSpans[$size][$order] ) ) {
    $multiple = self::COLUMN_WIDTH;
} else {
    $multiple = self::columnSpans[$size][$order];
}
```