Per-event ConCat reports

[glitchfur]

Currently, Tracker will use the CONCAT_BASE_URI environment variable to determine which ConCat instance to use when running reports that rely on it (specifically the Volunteer Applications and Application Department Summary reports). However, there is not a way to specify different ConCat instances for different events within Tracker. The same ConCat data will be pulled regardless of event chosen.

For example, ConCat typically archives previous year data for conventions by making it available at a subdomain (such as 2023.reg.example.com). Ideally, when having distinct con years as different "events" in Tracker, they should be able to pull reporting data from that year's ConCat URL rather than the current one.

The ConCat environment variables, including client ID and secret, should then only be used for logging into Tracker. Each event in Tracker should separately store a ConCat base URL, client ID and secret in the database to ensure it's pulling data from the correct source.