Provide option 'Pre-Release' on upload

**Current state:**
On upload, currently the nightly-checkbox can be used to upload nightly-builds. However, to upload pre-releases (alpha, beta, rc) a specific system of naming and upload is necessary, that is quite unintuitive and feels like it would overwrite the current released version. (And seems to be not really documented anywhere.)

**What would be more intuitive:**
Having on upload-window three radio-buttons for `release`, `pre-release`, `nightly`, and applying them correspondingly on the app-store-db & frontend would be much more easy & intuitive to provide pre-releases on the beta-channel to users.