feat(api): add citation_string as a CourtFilter filter field

The Bluebook's court abbreviations are fairly close to an identifier.
While there's a suggestion that they may not be stable[1], they are
still useful, especially when a caller is working from a citation.

[1] Fred Jacob, Even the Federal Government Can't Beat The Bluebook, 27 Green Bag 2d 117 (2024)

cl/search/filters.py

@@ -42,6 +42,7 @@ class Meta:
             "end_date": DATE_LOOKUPS,
             "short_name": ALL_TEXT_LOOKUPS,
             "full_name": ALL_TEXT_LOOKUPS,
+            "citation_string": ALL_TEXT_LOOKUPS
         }