WIP: Og image alt

[bkmgit]

This is a bug fix.
This is an enhancement or feature.

Summary

Adding alt text for open graph images

Context

Closes #3562

Further information

Changes are still needed to fit the current logic in https://github.com/mmistakes/minimal-mistakes/blob/master/_includes/seo.html These are

• add alt text for page_large_image

  • add alt text for page.header.og_image
  • add alt text for page.header.overlay_image
  • add alt text for page.header.image

• add alt text for page_teaser_image

  • add alt text for site_og_image
  • add alt text for page.header.teaser