Allow source searches to use underscores for spaces

[ToksT]

If a post source has spaces, like "example source", then it can't normally be searched with the source: metatag. You can type "source:example*source" as a workaround, but it would be better if "source:example_source" worked, interpreting the underscore as a space or an underscore.

[evazion]

Fixed in #2679.