update folder structure

[x] Change the top-level application script app.js to npg_sentry.js - to avoid collision when deployed globally
[x] Factor out all callbacks (app.get and app.post) from app.js into a controller module
[x] Change log message:
```
logger.info(`express started on port ${port}`);
```
on line 150 of app.js to 'npg_sentry started ..'
[x] In the controller, create a common dispatch function so that instead of repeating
```
res.status(200).json({ok: decision});
```
we can do
```
dispatch(res, payload);
```
This function can then have logging the payload for debugging purposes

wtsi-npg / npg_sentry