content [nfc]: Cut down TODOs on _launchUrl, following investigation

The second TODO, about the behavior of externalApplication, turns out
to be just how things work these days on Android:
  #74 (comment)

The first one, about the behavior of inAppWebView or platformDefault,
I've filed as #279.  I also added a number of details on the upstream
issue, and cross-linked it with a couple of other issues that would
be fixed by the same solution.

Author: gnprice

lib/widgets/content.dart

@@ -693,19 +693,9 @@ void _launchUrl(BuildContext context, String urlString) async {
   try {
     launched = await ZulipBinding.instance.launchUrl(url,
       mode: switch (Theme.of(context).platform) {
-        // TODO(upstream) The url_launcher default on Android is a weird UX:
-        //   opens a webview in-app, but on a blank black background.
-        //   The status bar is hidden:
-        //     https://github.com/flutter/flutter/issues/120883
-        //   but also there's no app bar, no location bar, no share button;
-        //   no browser chrome at all.
-        //   Probably what we really want is a "Chrome custom tab":
-        //     https://github.com/flutter/flutter/issues/18589
-        // TODO(upstream) With url_launcher's LaunchMode.externalApplication
-        //   on Android, we still don't get the normal Android UX for
-        //   opening a URL in a browser, where the system gives the user
-        //   a bit of UI to choose which browser to use:
-        //     https://github.com/zulip/zulip-flutter/issues/74#issuecomment-1514040730
+        // TODO(#279): On Android we settle for LaunchMode.externalApplication
+        //   because url_launcher's in-app is a weirdly bare UX.
+        //   Switch once that's fixed upstream (by us or otherwise).
         TargetPlatform.android => UrlLaunchMode.externalApplication,
         _ => UrlLaunchMode.platformDefault,
       },