Remove 'placeholder' image alt texts

[selfthinker]

If no image has been selected for news articles, a placeholder image gets added instead. And when that placeholder image is used, its alt text reads 'placeholder'.
That is bad for screen reader users as that content doesn't make sense. It's much better to leave it empty so screen readers can ignore it.

The build is currently failing but I don't have Whitehall set up, so cannot test or fix it. Can someone take this over and finish it (unless it's more complicated than I think)?

[gpeng]

Thanks @selfthinker. Fully agree with removing the alt text for placeholders.

I'm not sure this is the best way of handling the validation though. By allowing blank we're allowing all images to be uploaded with blank alt text. Could you update to only validate present if the image name isn't placeholder.jpg? That's still arguably a bit ugly coupling it all to the image name but it would achieve your goal and not break the validation for everything else. I think that will also fix the 3 failing tests you have.

You also don't really need Whitehall running on your machine. Nobody runs the full test suite locally in any case as it is too slow and CI will give you individual failures that you can then run locally. You'll just need mysql.

[tlwr]

I'm closing this PR as it has been open for a considerable amount of time and we're trying to cut down the number of orphaned PRs in alphagov.