Revert "Remove alternative text for images"

This reverts commit 9d5df8a53dd4d8d5696e78f0d549c4533ffa1753.

I misinformed Jack on the best behaviour here, prior art:
 - https://github.com/alphagov/frontend/pull/559
Merge pull request #653 from alphagov/fix-homepage-alt-text

Revert "Remove alternative text for images"