When disapproving an app, let me leave feedback

[danirabbit]

If we disapprove an app in manual review, we need to be able to give feedback to the developer about why.

The easiest thing to do would just be to open a new tab to https://github.com/USER/APP/issues/new when you hit the disapprove button and we can make it more self-contained later

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[cassidyjames]

If we wanted to keep it looking/functioning more like the automated issues being filed, we could provide a text box which gets filled in as the body of an issue filed by the AppCenter bot. Popping open the new tab would be quicker and dirtier, but I like aiming for consistent issues/labels/etc. if possible.