Add functions for setting description and OG meta

[jezen]

It's common that a website author will want to add a specific
description and Open Graph image to a given page in their website. These
functions are simple conveniences to add these meta tags to the document
head.

I decided against simply adding all possible meta tags, because not all
of them are useful, and even in the case of Open Graph tags, many of
them should be set only once and they should use the same value for the
entire website. In those cases, it's probably better for the website
author to add those tags in their layout template.

Closes #1659

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[snoyberg]

LGTM. CI discovered a compilation error, but once that's fixed this should be good to go.

[jezen]

@snoyberg Am I ok to squash and merge this?

[snoyberg]

Yes 👍