Various TypeScript improvements (#2309)

- Use Mithril.Attributes as base for ComponentAttrs, remove =any from class signature for Component
- Convert Alert to TypeScript, introduce AlertAttrs interface
- Convert AlertManagerState to TypeScript, add overload signatures for `show`, introduce AlertState interface for stored Alerts.
- Set ComponentAttrs as default T for Component
- Make attrs in AlertAttrs optional
- Add AlertIdentifier interface, simplify show type signature
- Remove mithril patch shim, as all patches onto m are now deprecated
- Use Mithril.Static for shim

js/src/common/components/Alert.tsx

@@ -1,31 +1,33 @@
-import Component from '../Component';
+import Component, { ComponentAttrs } from '../Component';
 import Button from './Button';
 import listItems from '../helpers/listItems';
 import extract from '../utils/extract';
+import Mithril from 'mithril';
+
+export interface AlertAttrs extends ComponentAttrs {
+  /** The type of alert this is. Will be used to give the alert a class name of `Alert--{type}`. */
+  type?: string;
+  /** An array of controls to show in the alert. */
+  controls?: Mithril.Children;
+  /** Whether or not the alert can be dismissed. */
+  dismissible?: boolean;
+  /** A callback to run when the alert is dismissed */
+  ondismiss?: Function;
+}
 
 /**
  * The `Alert` component represents an alert box, which contains a message,
  * some controls, and may be dismissible.
- *
- * ### Attrs
- *
- * - `type` The type of alert this is. Will be used to give the alert a class
- *   name of `Alert--{type}`.
- * - `controls` An array of controls to show in the alert.
- * - `dismissible` Whether or not the alert can be dismissed.
- * - `ondismiss` A callback to run when the alert is dismissed.
- *
- * All other attrs will be assigned as attributes on the DOM element.
  */
-export default class Alert extends Component {
-  view(vnode) {
+export default class Alert<T extends AlertAttrs = AlertAttrs> extends Component<T> {
+  view(vnode: Mithril.Vnode) {
     const attrs = Object.assign({}, this.attrs);
 
     const type = extract(attrs, 'type');
     attrs.className = 'Alert Alert--' + type + ' ' + (attrs.className || '');
 
     const content = extract(attrs, 'content') || vnode.children;
-    const controls = extract(attrs, 'controls') || [];
+    const controls = (extract(attrs, 'controls') || []) as Mithril.ChildArray;
 
     // If the alert is meant to be dismissible (which is the case by default),
     // then we will create a dismiss button to append as the final control in