Search: Use PostgreSQL websearch_to_tsquery

Now that PostgreSQL has been upgraded to version 12 (from 10), the
websearch_to_tsquery() function is available. This is much better for
general search purposes, and will add multiple capabilities.

I'm not sure if there's a full specification of what it supports
anywhere, but this is the main relevant section in the docs:
https://www.postgresql.org/docs/12/textsearch-controls.html

tildes/tildes/models/comment/comment_query.py

@@ -96,7 +96,9 @@ class CommentQuery(PaginatedQuery):
 
     def search(self, query: str) -> "CommentQuery":
         """Restrict the comments to ones that match a search query (generative)."""
-        return self.filter(Comment.search_tsv.op("@@")(func.plainto_tsquery(query)))
+        return self.filter(
+            Comment.search_tsv.op("@@")(func.websearch_to_tsquery(query))
+        )
 
     def only_bookmarked(self) -> "CommentQuery":
         """Restrict the comments to ones that the user has bookmarked (generative)."""

tildes/tildes/models/topic/topic_query.py

@@ -175,7 +175,7 @@ class TopicQuery(PaginatedQuery):
 
     def search(self, query: str) -> "TopicQuery":
         """Restrict the topics to ones that match a search query (generative)."""
-        return self.filter(Topic.search_tsv.op("@@")(func.plainto_tsquery(query)))
+        return self.filter(Topic.search_tsv.op("@@")(func.websearch_to_tsquery(query)))
 
     def only_bookmarked(self) -> "TopicQuery":
         """Restrict the topics to ones that the user has bookmarked (generative)."""