implement stop and start app buttons in UI

- [x] the buttons are implemented and wired to methods
in app.controller
- [ ] a service needs to be implemented
- [ ] need logic for determining if the app is currently
runnig to disable or enable the buttons

See #166 
@bajtos @anthonyettinger 
Most of the style is shared between `.btn-halfsize` and `.btn-fullsize`. Please refactor so that the shared settings are in one place only.