Upgrade react-native-webview to 3.x, 4.x, 5.x

[gnprice]

We're using react-native-webview version 2.0.0. In the months since that release, they've made several more with major changes.

In particular @jainkuniya found with git bisect that #3518 was introduced by switching from the RN-included WebView to react-native-webview. And he found that upgrading all the way to 5.9.1 fixes #3518. (Described on #3536). AFAIK he didn't test versions in between, so it's possible a much earlier version already fixes that issue. Anyway, we should get to the latest.

The latest version of react-native-webview, from last week, comes as a pair (see NPM): 6.3.1 with AndroidX, and 5.12.1 without. Until #3548 , we'll want the non-AndroidX version, so that means the latest 5.x version.

@jainkuniya also described on #3536 finding that upgrading to 5.9.1 wasn't smooth. So let's do this in steps:

• First, to the latest 3.x.
• Then the latest 4.x. -> looking at the version history, 5.0.1 came less than 3 days after 4.0.0, so perhaps skip this.
• Then the latest 5.x.

For each version:

• Read the release notes. Figure out what breaking changes the maintainers say they made.
• Try the upgrade.
• Fix any issues.
• In the commit message, include:
  • a link to the release notes
  • a mention of any breaking changes that we think might matter
  • a description of what you did about them.

A couple of things we know about already:

• @jainkuniya mentioned on dependency: Upgrade react-native-webview to ^6.0.0. #3536 something changed in 5.x with postMessage, and linked to postMessage does not work in iOS but in Android after switching from react native core to react-native-webview react-native-webview/react-native-webview#363 (comment) .
• That thread links in turn to another thread: [DISCUSS] Cross platform API for RN<>WebView 2-way RPC/message passing react-native-webview/react-native-webview#66 (comment)
• In short it looks like postMessage has been dropped in favor of an API that's said to be better. We should switch to that new API.

[gnprice]

This appears to be a requirement for the RN v0.59 upgrade, #3399 -- versions of react-native-webview prior to 5.x say they're only compatible with RN v0.57. (A bit more discussion in #3561 .) Bumping the priority accordingly.

[gnprice]

A tactical note: the 5.0.0 release notes say:

"""
Side note: if you wish to keep compatibility with the old version when you upgrade, you can use the injectedJavascript prop to do that:

const injectedJavascript = `(function() {
  window.postMessage = function(data) {
    window.ReactNativeWebView.postMessage(data);
  };
})()`;

"""

So that sounds like the right solution to pursue for now, to unblock the upgrade. Then we can switch to the new, said-to-be-better API at our leisure.