Split up bodyToHtml (#12840)

* Split up bodyToHtml

This (very incrementally) splits up the bodyToHtml function to avoid
the multiple return types and hopefully make it a touch easier to
comprehend. I'd also like to see what the test coverage says about
this, so this is is somewhat experimental. This shouldn't change
any behaviour but the comments in this function indiciate just how
subtle it is.

* Remove I prefix

* Missed emoji formatting part

src/utils/MessageDiffUtils.tsx

@@ -22,7 +22,7 @@ import { IContent } from "matrix-js-sdk/src/matrix";
 import { logger } from "matrix-js-sdk/src/logger";
 import { unescape } from "lodash";
 
-import { bodyToHtml, checkBlockNode, IOptsReturnString } from "../HtmlUtils";
+import { bodyToHtml, checkBlockNode, EventRenderOpts } from "../HtmlUtils";
 
 function textToHtml(text: string): string {
     const container = document.createElement("div");
@@ -31,9 +31,8 @@ function textToHtml(text: string): string {
 }
 
 function getSanitizedHtmlBody(content: IContent): string {
-    const opts: IOptsReturnString = {
+    const opts: EventRenderOpts = {
         stripReplyFallback: true,
-        returnString: true,
     };
     if (content.format === "org.matrix.custom.html") {
         return bodyToHtml(content, null, opts);