linter: Add rule against using inline event handlers.

docs/code-style.md

@@ -308,11 +308,6 @@ Don't use the tag name in a selector unless you have to. In other words,
 use `.foo` instead of `span.foo`. We shouldn't have to care if the tag
 type changes in the future.
 
-Don't use inline event handlers (`onclick=`, etc. attributes). Instead,
-attach a jQuery event handler
-(`$('#foo').on('click', function () {...})`) when the DOM is ready
-(inside a `$(function () {...})` block).
-
 ### Python
 
 -   Scripts should start with `#!/usr/bin/env python3` and not

tools/linter_lib/custom_check.py

@@ -430,6 +430,13 @@ def build_custom_checkers(by_lang):
         {'pattern': '\Walt=["\']{{ ?["\']',
          'description': "alt argument should be enclosed by _().",
          },
+        {'pattern': r'\bon\w+ ?=',
+         'description': "Don't use inline event handlers (onclick=, etc. attributes) in HTML. Instead,"
+                        "attach a jQuery event handler ($('#foo').on('click', function () {...})) when "
+                        "the DOM is ready (inside a $(function () {...}) block).",
+         'exclude': set(['templates/zerver/dev_login.html']),
+         'good_lines': ["($('#foo').on('click', function () {}"],
+         'bad_lines': ["<button id='foo' onclick='myFunction()'>Foo</button>", "<input onchange='myFunction()'>"]},
     ]  # type: RuleList
     handlebars_rules = html_rules + [
         {'pattern': "[<]script",