Recommend "Get on AppCenter" button

AppCenter currently supports `appstream://` links to AppCenter apps. I think this function could be used in a "Get on AppCenter" button like the Google Play button or the iOS App Store button.

I made a sample implementation at https://github.com/jepotter1-archive/appcenter-button. Such a button could be recommended to developers after they finish uploading their app so they can then post an AppCenter link to their GitHub, website, etc.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/45581849-recommend-get-on-appcenter-button?utm_campaign=plugin&utm_content=tracker%2F27692469&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F27692469&utm_medium=issues&utm_source=github).

Review the issue description and the sample implementation at https://github.com/jepotter1-archive/appcenter-button first, then locate where the developer dashboard finishes an app upload. Done means developers are offered a "Get on AppCenter" button that links to the app's appstream:// URL for sharing on GitHub or a website.

Written by the indexing model from the issue text.