Let TwitterImage type depend on image size

[IreneStr]

As discussed in the UX channel, we want to let the size of the uploaded image determine what type of TwitterImage we show.

Docs about summary card with large image:

Images for this Card support an aspect ratio of 2:1 with minimum dimensions of 300x157 or maximum of 4096x4096 pixels.

To do:

• Images with minimum dimensions of 300x157 should result in a summary-large-image type.
• Smaller images should result in a summary type.

NB.

• Don't worry about the aspect ratio. If the aspect ratio is far off, that would result in a strange preview which should prompt the user to use another image.
• We'll deal with too large images (> 4096x4096) in a later stage.
• The summary-large-image card itself has an 506x254 image. That means that images between 300x157 and 506x254 are going to be stretched out in Twitter. We want to replicate this behavior in our preview. Later, we will add a notification with a warning in those cases.

Parent #28

[IreneStr]

Closing in favor of #209