Stop search engines indexing contentless pages

[WilliamWDTK]

I've noticed on a couple of occasions that Google returns search results for annotation and similar request pages. In the case of the former, it adds no useful content, and I can't see that the similar requests page is useful to be indexed and returned as a search result. Additionally, it may just be another thing to be anonymised when people request erasure.

We could consider this for the below, if not already done:

• The annotation page (/request/SLUG/annotate)
• Any links that always require a sign-in (reply, report, status update, request ZIP download)
• The similar requests page (/request/SLUG/similar)

[HelenWDTK]

Linked to #8132

[garethrees]

Closing in favour of #8132