Add a "private" filter on the reuses endpoint

[magopian]

This is a new take on #1397

In this PR, we'll have the following behavior:

1. always return each and every reuse available to the logged in user (if any), including private, deleted and archived ones
2. if the filter is false, eg private=False, only return the reuses that don't have this filter (no private reuses)
3. if the filter is true, eg private=True, only return the reuses that have this filter (only private reuses)