[e.cash] Add custom metadata for blog posts

Summary: Adding some logic so every blog post can use the post content to populate metadata with a title, description, and preview image

Test Plan: preview and check the meta data on blog post pages

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D18789