eslint: Add key-spacing linter rule.

Apparently, we didn't have one of these, and thus had a moderate
number of generally very old violations in the codebase.  Fix this and
clear the ones that exist..

.eslintrc.json

@@ -245,6 +245,12 @@
             "FunctionExpression": {"parameters": "first"},
             "FunctionDeclaration": {"parameters": "first"}
         }],
+        "key-spacing": [ "error",
+            {
+                "beforeColon": false,
+                "afterColon": true
+            }
+        ],
         "keyword-spacing": [ "error",
             {
                 "before": true,

frontend_tests/node_tests/message_list.js

@@ -269,7 +269,8 @@ run_test('last_sent_by_me', () => {
 
 run_test('local_echo', () => {
     var list = new MessageList({});
-    list.append([{id:10}, {id:20}, {id:30}, {id:20.02}, {id:20.03}, {id:40}, {id:50}, {id:60}]);
+    list.append([{id: 10}, {id: 20}, {id: 30}, {id: 20.02},
+                 {id: 20.03}, {id: 40}, {id: 50}, {id: 60}]);
     list._local_only = {20.02: {id: 20.02}, 20.03: {id: 20.03}};
 
     assert.equal(list.closest_id(10), 10);