Search can return invalid links
https://forum.nim-lang.org/search?q=graphql

produces some links that are invalid, for instance clicking on the response from dom96 produces

    Couldn't retrieve posts.

    Please report this issue to us so we can fix it!

I can think of a few approaches to address this:

* a maintenance sql report that identifies anomalies in the db followed by correction
* an improved sql query in the search that excludes invalid matches
* filter the search results using SearchResult.isValid
* filter the search results using SearchResult.url().isValid

where isValid does the obvious check.

What do you think?