SVG-SS4

Add the following methods to your `Image` subclass to allow SVG uploads via `$has_one` `UploadField`'s and uploads via the Asset Admin.

Hat-tip to https://github.com/restruct/silverstripe-svg-images. All I've done is:

* Removed superfluous SVG template logic
* Called `isSVG()` from everywhere
* Added comments where appropriate
* Put all necessary YML config into one place

YML config:

# SVG Capabilities
SilverStripe\Assets\File:
  app_categories:
    image:
      - svg
    image/supported:
      - svg
  allowed_extensions:
    - 'svg'
  class_for_file_extension:
    # Unknown at this time, whether this is even needed
    svg: My\Image\Subclass

Image subclass:

    /**
     * @return string
     */
    public function getFileType()
    {
        if ($this->isSVG()) {
            return 'SVG image - good for line drawings';
        }

        return parent::getFileType();
    }

    /**
     * @param  string $dim
     * @return mixed array|string
     */
    public function getDimensions(string $dim = 'string')
    {
        if (!$this->isSVG() || !$this->exists()) {
            return parent::getDimensions($dim);
        }

        if (!$this->getField('Filename')) {
            return [];
        }

        $filePath = $this->getFullPath();

        // parse SVG
        $out = new DOMDocument();
        $out->load($filePath);

        if (!is_object($out) || !is_object($out->documentElement)) {
            return [];
        }

        // Get dimensions from viewbox or else from width/height on root svg element
        $root = $out->documentElement;

        if ($root->hasAttribute('viewBox')) {
            $vbox = explode(' ', $root->getAttribute('viewBox'));
            $size[0] = $vbox[2] - $vbox[0];
            $size[1] = $vbox[3] - $vbox[1];
        } else if ($root->hasAttribute('width')) {
            $size[0] = $root->getAttribute('width');
            $size[1] = $root->getAttribute('height');
        } else {
            return ($dim === 'string') ? 'No size set (scalable)' : 0;
        }

        // (regular logic/from Image class)
        return ($dim === 'string') ? "$size[0]x$size[1]" : $size[$dim];
    }

    /**
     * Scale image proportionally to fit within the specified bounds
     *
     * @param  int $width The width to size within
     * @param  int $height The height to size within
     * @return AssetContainer
     */
    public function Fit($width, $height)
    {
        if ($this->isSVG()) {
            return $this;
        }

        // Forward to regular Image class
        return parent::Fit($width, $height);
    }

    /**
     * @param  int $width
     * @param  int $height
     * @return AssetContainer
     */
    public function FitMax($width, $height)
    {
        if ($this->isSVG()) {
            return $this;
        }

        // Forward to regular Image class
        return parent::FitMax($width, $height);
    }

    /**
     * Called from {@link ThumbnailGenerator::generateThumbnail()}
     *
     * @return AssetContainer
     */
    public function existingOnly()
    {
        if ($this->isSVG()) {
            return $this;
        }

        return parent::existingOnly();
    }

    /**
     * @return bool
     */
    public function isSVG()
    {
        return $this->getExtension() === 'svg';
    }