Create small app to track link-throughs

## Problem

We have a Google Calendar. We have no idea how much it's used. Google Calendar has no way to track calendar subscibers.

## Potential Solution

We could create a small app that allows arbitrary links to be tracked via pass-through. Since each event in the calendar has a link in the description (with details not available from the ical event), that could proxy for how much use the calendar is getting.

So for example, if we had an event with https://example.com in the description, the ical event could link to something like `https://my-link-tracker-app.herokuapp.com?url=https://example.com`

This link tracker app would then write data whenever someone using it as a pass-through. This could be either in a spreadsheet, or in Google Analytics.

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Note that the approach would require putting very specific links into the calendar, which is a pain, and would probably be forgotten. We could instead create another app that takes an ical feed and rewrites all the links to have the right format. Then we could add that ical feed to another gcal via importing the rewritten ical feed, and embedding it

## Alternative Considered

- maybe a service allows for tracking ical links

## Related

- https://github.com/alphagov/external-link-tracker