apply global cert to existing app

[AubreyHewes]

It should be possible to apply a global cert to an existing app.

Currently a global cert is only applied to new apps.

So something like global-cert:apply <app> should apply the default global cert to the given app. This would also overwrite a possible other cert on the app.

For example:

app1 has an le-cert (dokku-letsencrypt) but I want to apply the global cert. Currently this would entail manual steps; removing the tls files and copying/symlinking from the global-cert

Usable PR depends on #4