Enable searching for packages with topics with `#<topic-name>`

[parlough]

Since topics are presented as hashtags, I believe users expect to be able to search for them in the same way since other platforms with hashtags do as well.

It's otherwise not obvious how to search for a topic (topic:<name>), as it's not an option in the left form. Users will only learn by reading the documentation or by clicking on a topic. It should be instantly intuitive and relying on the well-known search behavior of hashtags from other platforms should provide that :)

I would expect specifying more than one would topic would require them all to be specified by the listed packages. That way users can dive down into a topic subcategory. For example #server #html.

This should be verified to not break quoted "#<topic name>" literal text search.

[parlough]

\cc @szakarias For your thoughts :)

[szakarias]

Great idea!