webview: Use native scroll deceleration on iOS

[titanous]

The default WebView decelerationRate is fast, which slows down momentum scrolling through messages significantly. This changes it to normal which is the native rate on iOS and makes it behave like other apps, allowing for much faster scrolling.

Test Plan: confirmed that this has the expected scroll momentum on an iPhone 12.

Fixes: #3557

[chrisbobbe]

Good find, thanks for the PR! This fixes #3557, a long-standing issue. 🎉 Merged, with a few tweaks:

• I checked out the branch and ran tools/test (this has stopped running automatically for PRs and posting the results on GitHub; that's Migrate to GitHub Actions (from Travis CI) #4174), and ESLint told me it would like double quotes there instead of singled quotes; fixed.
• Added Fixes: #3557 to the commit message, and the post at the top of this thread, so that issue gets linked up and closed automatically. 🎉
• Fixed a commit-message style nit by changing "This changes it" to "Change it"; one obscure difference (we should probably document it somewhere) between the mobile-app project and the main Zulip commit-message style guide (here) is that we explicitly prefer the imperative in the commit message body, not just the summary line, for describing what work is done in the commit. The https://github.com/zulip/zulip project itself is inconsistent on this point; Greg discusses that at Store server feature level in Redux #4079 (comment).