lint: Ban general use of user_profile.save().

This often can cause minor caching problems.

Obviously, it'd be better if we had access to the AST and thus could
do this rule for UserProfile objects in general.

tools/linter_lib/custom_check.py

@@ -241,6 +241,14 @@ def build_custom_checkers(by_lang):
          'description': '@login_required is unsupported; use @zulip_login_required',
          'good_lines': ['@zulip_login_required', '# foo @login_required'],
          'bad_lines': ['@login_required', ' @login_required']},
+        {'pattern': '^user_profile[.]save[(][)]',
+         'description': 'Always pass update_fields when saving user_profile objects',
+         'exclude_line': set([
+             ('zerver/lib/actions.py', "user_profile.save()  # Can't use update_fields because of how the foreign key works."),
+         ]),
+         'exclude': set(['zerver/tests', 'zerver/lib/create_user.py']),
+         'good_lines': ['user_profile.save(update_fields=["pointer"])'],
+         'bad_lines': ['user_profile.save()']},
         {'pattern': '^[^"]*"[^"]*"%\(',
          'description': 'Missing space around "%"',
          'good_lines': ['"%s" % ("foo")', '"%s" % (foo)'],