Add meta descriptions for download pages

[ocean90]

In #3673 new templates were added but those aren't handled in custom_open_graph_tags() yet, which results in a "Visit the post for more." description. See ​https://en-au.wordpress.org/download/ as an example. (The main site doesn't show them at all since Jetpack isn't set up.)

Related: [7418], #1665

[jonoaldersonwp]

Should probably take some inspiration from how we handled this: https://meta.trac.wordpress.org/ticket/3940

[ocean90]

Pinged #marketing for help: ​https://wordpress.slack.com/archives/C0GKJ7TFA/p1548497927711900

[jonoaldersonwp]

Hold up, do we mean meta descriptions, or og:description values? And/or, are we not differentiating between them (NB; we should be)?

[jonoaldersonwp]

Text approved by @joostdevalk:

Download WordPress today, and get started on creating your website with one of the most powerful, popular, and customizable platforms in the world.

Localised/translated versions should try and remain as faithful to that as possible.

[ocean90]

I guess that's for /download/? Still need the ones for the three sub pages.

[jonoaldersonwp]

/download/releases/
Browse and download previous versions of WordPress for testing and development.

/download/beta-nightly/
Get the latest, unstable or work-in-progress versions of WordPress for testing and development.

/download/source/
See how WordPress works under the hood, and contribute your own code to the world's most populare content management system!

[ocean90]

In 8182:

Main: Add meta descriptions for download pages.

Props jonoaldersonwp.
Fixes #4085.