Img helper size config

[allouis]

refs #10181
Given a theme with a package.json with an image_sizes object like this:
(N.B. the small key must have at least a height or a width, or both)

{
  "config": {
    "image_sizes": {
      "small": {
        "width": 300,
        "height": 200
      },
      "wide": {
        "width": 2000
      },
      "tall": {
        "height": 2000
      }
    }
  }
}

Using the img_url helper in the theme like

<img src="{{img_url profile_image}}">
<img src="{{img_url profile_image size="small"}}">
<img src="{{img_url profile_image size="wide"}}">
<img src="{{img_url profile_image size="tall"}}">
<img src="{{img_url profile_image size="unknown-size!!!"}}">

Will render to

<img src="/content/images/path/to/profile_image.ext">
<img src="/content/images/size/w300h200/path/to/profile_image.ext">
<img src="/content/images/size/w2000/path/to/profile_image.ext">
<img src="/content/images/size/h2000/path/to/profile_image.ext">
<img src="/content/images/path/to/profile_image.ext">

If the package.json of the theme does not define the image_sizes object or the requested size is not included in the image_sizes object, the original image will be served.

[ErisDS]

Additional note this PR has the ability to specify only widths/heights which conflicts with the spec and needs confirmation

[ErisDS]

We've agreed it is preferable to allow either width, height or both.

[allouis]

I've updated the PR description to reflect the new url structure i.e wXhX, wX and hX

[allouis]

@ErisDS This has been updated to work with absolute urls when they belong to the blog domain. Which means v2 content api compat ☺️