sgrep: Install syntactic code search tool as an external linter.

Add sgrep (sgrep.dev) to tooling and include simple rule as
proof of concept. Included rule detects use of old django render
function.

Also added a rule that looks for if-else statements where both
code paths are identical.

tools/linter_lib/custom_check.py

@@ -415,9 +415,6 @@ python_rules = RuleList(
          'description': "Don't use datetime in backend code.\n"
          "See https://zulip.readthedocs.io/en/latest/contributing/code-style.html#naive-datetime-objects",
          },
-        {'pattern': r'render_to_response\(',
-         'description': "Use render() instead of render_to_response().",
-         },
         {'pattern': 'from os.path',
          'description': "Don't use from when importing from the standard library",
          },