Adding and deleting co-maintainers

[BernhardPosselt]

An app owner should be able to add and remove co-maintainers to and from his app. The add co-maintainer button should be under the co-maintainer list on the app detail view, the remove co-maintainer button should be to the right or left of the co-maintainer name. Both button types should only be visible for app owners.

We probably want autocompletion for the form and the process should be similar to the way that Github does it (check out adding collaborators)