Fix SEO text for some pages

Summary:
Page previews on Telegram, Google search, Twitter, etc. rely on meta
tags. Some page previews are a mess of code since there is no text-based content
in the first few lines of the file. We can override this automatic behavior for
the pages that need it. The legacy blog posts appear to render correctly in
Google searches, so no fix is necessary for those pages.

This patch also fixes up the meta description used across some pages where the
site title ends up duplicated (see https://www.google.com/search?q=bitcoinabc.org for example)

Depends on D7626 for sanity

Test Plan:
make serve
Examine meta tags on homepage, release page, and legacy annoucements page.
Verify meta tags still appear as expected for legacy posts.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7627