Allow implicit and explicit definition of the default image post

[poppastring]

The main image of a blog post is implicitly defined as the first image found in the HTML (see code here), we could make this more explicit in certain scenarios.

My suggestion is to ensure that the image can be identified by a pre-defined class name as follows:
<img class="hero-image" src="/someimge.jpg" >

The code should find this image regardless of where it is in the post and present it as the hero image.

[clemensv]

The post already has a data field for the URL. That data field should be filled with the upload from the edit page if empty, but should also be settable to an external URL (e.g. Flickr) that might not even be embedded in the post text and then is only used as the teaser image, e.g. in Twitter cards.

I would also like a second image URL that can be optionally set for a page banner if the template wants/needs it.