Add option to install apps in nextcloud (using "badges")

## Description:

As already discussed in https://github.com/nextcloud/news/pull/318, it would be great to have the option to install apps in your nextcloud right away. Right now when the user wants to install an app he found in the [App Store](https://apps.nextcloud.com/) or on GitHub, the user has to open his/her nextcloud, open the build in app store and search for the app there.

It would be great for developers to create a so called [Badge](https://play.google.com/intl/en_us/badges/) for their app (e.g. on their GitHub Project page) which will install the app in the users nextcloud installation. This badge can then be integrated into the official nextcloud app store as well. I created a small mockup of the workflow: (Please note that the url for the second step should be something like https://apps.nextcloud.com/...)

As already said in the referenced PR, this will be a huge improvement in the user experience as it will make it much easier for users to install other apps in their nextcloud. Especially considering the recent focus on widening the user base simplifying the installation process makes much sense here. It also allows developers to advertise their apps more/better since it's basically a "one-click" installation.

Quote by @jancborchardt:
> Then your app store account could also store the address of your Nextcloud, and the install button would then be direct for every install after the first!

## Workflow: