Allow to install an application by providing its manifest URL

[clochix]

For now, we can only install application from Github, because we need to guess the absolute URL of the manifest from the repository URL.
We could propose an alternative path: ask the user the absolute URL of the manifest, and add to the manifest the URL of the repository.
By saving for each application tu URLs of the repository and of the package, we would be able to install applications from everywhere.