Allow developers to manage categories/tags

[viktorix]

Right now, developers can select categories/tags during initial submission. If they need to change it for some reason, there's no way to do it on their own. For now, we can tell them to message us and we will make the change.

We should consider in the future an option that allows them to manage this themselves. We could add a simple "Manage" button next to "Update Download Link" button and offer a simple form (maybe as a modal) to change categories or tags.