Add a warning when capitalized booleans appear in query

The warning from signac.filterparse._cast is burried in the shell and
automatically searching for capitalized boolean string is likely not
the expected result.

signac_dashboard/dashboard.py

@@ -373,6 +373,8 @@ def _job_search(self, query):
                 try:
                     f = json.loads(query)
                 except json.JSONDecodeError:
+                    if "True" in query or "False" in query:
+                        flash("JSON booleans are lowercase", "warning")
                     query = shlex.split(query)
                     f = signac.filterparse.parse_filter_arg(query)
                     flash(f"Search string interpreted as '{json.dumps(f)}'.")