Cross-community search

[cellio]

Prompted by some Discord chat and also in part by meta:291073 (for which this would be a prerequisite). See also meta:289142 and meta:294699.

Communities sometimes have overlapping or related scope, and "combine the communities" or "build bigger walls" isn't generally what we want to do. Instead, it would be helpful if users could search not only within the current community but also across the entire network.

I think a minimum viable solution would be:

• Add something to the search syntax to request network-wide search. We currently default to a community and you can use category:N to restrict downward; this would be something in the other direction, to say "not just this community". This new option would be incompatible with category; you can't both choose the whole network and restrict to a category.
• In search results, show (somehow) the community that a post came from, either always (probably easier) or if it's not the local one (where you did the search). This probably means another "badge" alongside the category in search results.
• Make sure to test with the filter widget on the results page to make sure nothing bad happens (particularly with included/excluded tags).

[MoshiKoi]

Duplicate of #1255 ?

[cellio]

Whoops.