Handle missing width and height attributes for images

[mjangda]

Not all <img> tags have width/height attributes which are required for the matching <amp-img> tag. We should find a way to include that somehow.

• If srcset or sizes are included, try to pull from them, if possible.
• Pull from attachment metadata.
• Fetch the remote image and calculate/save the size (getimagesize and wpcom's getimagesize).

Related #13

[mjangda]

This is partially handled. There are cases like hotlinked images that we need to figure out. Possible options:

• live with the validation errors/potentially broken layouts
• fetch the remote image and calculate the size
• use something like Photon to get dimension data (Photon doesn't support this but we could add it)

The third one is only workable for wpcom and self-hosted sites with Jetpack.

[mjangda]

We should set a max of 600 ($content_width if set, or fallback to 600) for images since that's the max size of our column.