Allowing the hoster to disable the Android/iOS web app `<meta>` tags.

I'd like to give users the option to configure whether their hosted selfoss includes the `<meta>` tags that enable web app support, while retaining them as the default behavior.  This has been a major annoyance for me, as I primarily read on Android with Chrome; however, this is *not* really a bug in selfoss, but rather in Android Chrome.

Because selfoss was inserting the `<meta name="mobile-web-app-capable" content="yes">` tags, Chrome's "Add to Homescreen" behavior treated it as a full-screen web app.  This would typically be excellent, but on recent versions of Android Chrome, after opening a few articles to read the full content, I receive the error "Unsupported number of Chrome instances" and have to exit both the full-screen selfoss web app and my other running Chrome instance before I am able to continue reading.