lint: Add linter rule banning $.get and friends.

These are not allowed in our style guide.

docs/contributing/code-style.md

@@ -217,7 +217,7 @@ code a lot uglier, in which case it's fine to go up to 120 or so.
 When calling a function with an anonymous function as an argument, use
 this style:
 
-    $.get('foo', function (data) {
+    my_function('foo', function (data) {
         var x = ...;
         // ...
     });

tools/linter_lib/custom_check.py

@@ -230,6 +230,18 @@ def build_custom_checkers(by_lang):
          'description': "`Use $(f) rather than `$(document).ready(f)`",
          'good_lines': ['$(function () {foo();}'],
          'bad_lines': ['$(document).ready(function () {foo();}']},
+        {'pattern': '[$][.](get|post|patch|delete|ajax)[(]',
+         'description': "Use channel module for AJAX calls",
+         'exclude': set([
+             # Internal modules can do direct network calls
+             'static/js/blueslip.js',
+             'static/js/channel.js',
+             # External modules that don't include channel.js
+             'static/js/stats/',
+             'static/js/portico/',
+         ]),
+         'good_lines': ['channel.get(...)'],
+         'bad_lines': ['$.get()', '$.post()', '$.ajax()']},
         {'pattern': 'style ?=',
          'description': "Avoid using the `style=` attribute; we prefer styling in CSS files",
          'exclude': set([