Use `replace` in bang query formatting

Using `format` for formatting bang queries caused a KeyError for some
searches, such as !hd (HUDOC). In that example, the URL returned in the
bangs json was `http://...#{%22fulltext%22:[%22{}%22]...`, where
standard formatting would not work due to the misidentification of
"fulltext" as a formatting key.

The logic has been updated to just replace the first occurence of "{}"
in the URL returned by the bangs dict.

Fixes #513

app/utils/bangs.py

@@ -58,7 +58,7 @@ def resolve_bang(query: str, bangs_dict: dict) -> str:
         if operator not in split_query \
                 and operator[1:] + operator[0] not in split_query:
             continue
-        return bangs_dict[operator]['url'].format(
+        return bangs_dict[operator]['url'].replace('{}',
             query.replace(operator if operator in split_query
-                          else operator[1:] + operator[0], '').strip())
+                          else operator[1:] + operator[0], '').strip(), 1)
     return ''