[e.cash] Adjust blog featured image

Summary:
Adjusting blog pages featured images to show entire image instead of cover.
Since not all the images have the same aspect ratio, need to opt for a normal img element and simply display it as full width and auto height

Test Plan: preview and check

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18748