Add support for external integrations

External integrations are just like apps, but they don't have releases, you can update them via the web and they won't be shown in the internal Nextcloud app store.

- [x] - Register an integration
- [x] - List all owned integrations
- [x] - Edit existing integration
- [ ] - Code cleanup
- [ ] - Documentation
- [ ] - Tests
- [ ] - Sample data
- [ ] - Set User/developer/admin documentation fields
- [ ] - Add multiple screenshots (currently only one can be added)
- [ ] - Add multiple links
- [ ] - Add ability to transfer integrations between owners / remove them from transfer page for now
- [ ] - Send email on approved submission / Other feedback channel (waiting for feedback etc)
- [ ] - Code cleanup