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

@@ -0,0 +1,50 @@
+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.
+ */
+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') || []) 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
+    // the alert.
+    const dismissible = extract(attrs, 'dismissible');
+    const ondismiss = extract(attrs, 'ondismiss');
+    const dismissControl = [];
+
+    if (dismissible || dismissible === undefined) {
+      dismissControl.push(<Button icon="fas fa-times" className="Button Button--link Button--icon Alert-dismiss" onclick={ondismiss} />);
+    }
+
+    return (
+      <div {...attrs}>
+        <span className="Alert-body">{content}</span>
+        <ul className="Alert-controls">{listItems(controls.concat(dismissControl))}</ul>
+      </div>
+    );
+  }
+}