Add goal tracking to the export feature

[raae]

When the user clicks the export button it should be tracked.

1. Add GATSBY_FATHOM_EXPORT to your .env.development (any value will due)
2. Export FATHOM_EXPORT from ./constants
3. Import the trackGoal function from the tracking feature.
4. Import FATHOM_EXPORT from ./constants
5. When button is clicked call trackGoal with FATHOM_EXPORT as param
6. Test and make sure the console logs out Track fathom goal <FATHOM_EXPORT>)

I will add a real value to Netlify env variables so this will be tracked properly by fathom in production.

[olavea]

Question about
5. When button is clicked call trackGoal with FATHOM_EXPORT as param
A. Should trackGoal be called after openSaveFileDialog? or
B. Should trackGoal be called before openSaveFileDialog?

I am currently going with "B", but you're the POW! Queen @raae 👑 so it's your call.

[raae]

Let's do before, as if it fails the intent of the user is still to export. Later we can add some error tracking. Ie B as you suggested.