feat: make social links configuration optional

[wotnak]

Resolves https://github.com/Lullabot/gatsby-theme-adr/issues/59. Allows providing empty value for socialLinks configuration option or to skip it entirely.

[e0ipso]

Code looks good, and visual inspection looks good in Tugboat.

Thanks for the contribution!

[github-actions[bot]]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: