Move fetch-blog-feed script to more appropriate location

Summary: This script doesn't belong at the top-level.

Test Plan:

scripts/fetch-blog-feed.sh
ls -al _data    # verify blog-feed.json is up-to-date

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7432