Enable @typescript-eslint/explicit-function-return-type in /src (#9788)

* Enable `@typescript-eslint/explicit-member-accessibility` on /src

* Prettier

* Enable `@typescript-eslint/explicit-function-return-type` in /src

* Fix types

* tsc strict fixes

* Delint

* Fix test

* Fix bad merge

src/components/views/toasts/GenericExpiringToast.tsx

@@ -38,7 +38,7 @@ const GenericExpiringToast: React.FC<IProps> = ({
     toastKey,
     numSeconds,
 }) => {
-    const onReject = () => {
+    const onReject = (): void => {
         if (onDismiss) onDismiss();
         ToastStore.sharedInstance().dismissToast(toastKey);
     };

src/components/views/toasts/NonUrgentEchoFailureToast.tsx

@@ -22,11 +22,11 @@ import Modal from "../../../Modal";
 import ServerOfflineDialog from "../dialogs/ServerOfflineDialog";
 
 export default class NonUrgentEchoFailureToast extends React.PureComponent {
-    private openDialog = () => {
+    private openDialog = (): void => {
         Modal.createDialog(ServerOfflineDialog, {});
     };
 
-    public render() {
+    public render(): JSX.Element {
         return (
             <div className="mx_NonUrgentEchoFailureToast">
                 <span className="mx_NonUrgentEchoFailureToast_icon" />

src/components/views/toasts/VerificationRequestToast.tsx

@@ -54,7 +54,7 @@ export default class VerificationRequestToast extends React.PureComponent<IProps
         this.state = { counter: Math.ceil(props.request.timeout / 1000) };
     }
 
-    public async componentDidMount() {
+    public async componentDidMount(): Promise<void> {
         const { request } = this.props;
         if (request.timeout && request.timeout > 0) {
             this.intervalHandle = window.setInterval(() => {
@@ -83,20 +83,20 @@ export default class VerificationRequestToast extends React.PureComponent<IProps
         }
     }
 
-    public componentWillUnmount() {
+    public componentWillUnmount(): void {
         clearInterval(this.intervalHandle);
         const { request } = this.props;
         request.off(VerificationRequestEvent.Change, this.checkRequestIsPending);
     }
 
-    private checkRequestIsPending = () => {
+    private checkRequestIsPending = (): void => {
         const { request } = this.props;
         if (!request.canAccept) {
             ToastStore.sharedInstance().dismissToast(this.props.toastKey);
         }
     };
 
-    public cancel = () => {
+    public cancel = (): void => {
         ToastStore.sharedInstance().dismissToast(this.props.toastKey);
         try {
             this.props.request.cancel();
@@ -105,7 +105,7 @@ export default class VerificationRequestToast extends React.PureComponent<IProps
         }
     };
 
-    public accept = async () => {
+    public accept = async (): Promise<void> => {
         ToastStore.sharedInstance().dismissToast(this.props.toastKey);
         const { request } = this.props;
         // no room id for to_device requests
@@ -148,7 +148,7 @@ export default class VerificationRequestToast extends React.PureComponent<IProps
         }
     };
 
-    public render() {
+    public render(): JSX.Element {
         const { request } = this.props;
         let description;
         let detail;