Refactor exporting action to be site-wide

[SaltyQuetzals]

Is your feature request related to a problem? Please describe.
We currently re-implement the "export as CSV" action in both the Rsvp and Application ModelAdmins, without re-using code.

Describe the solution you'd like
An implementation similar to the one described on the Django admin actions documentation. We can essentially re-use the code we already have for exporting data as CSV, but reduce the amount of code we need to export (and improve our codebase) by refactoring exporting into an intermediate view that simply returns a CSV.

Additional context
This feature is not necessary to launch, but is an opportunity for improving our codebase.