Allow get invites endpoint to return all invites filtered by status

[virtuous-cat]

[Currently the get /realms/realm_id/invites/ endpoint is setup to only return the pending invites for the realm.]

The "right" way of doing it would be to return all the invites (with pagination), then have a status get param which allows to filter by status. For example, our request could be:

/realms/v0/invites?status=PENDING

to only get the pending invites.

Two things that I didn't add in the design that prevents us from doing this the right way:

1. We don't have a status enum we return with the invite. This is because I thought we'd just calculate it with expired_at and didn't want to duplicate the information, but I was wrong. We should have a status field which has at least: PENDING, ACCEPTED, EXPIRED (potentially for the future REVOKED).
2. We don't have pagination for this endpoint, which is not a huge deal for now if we're returning only pending invites, but will be at some point in the future.

Originally posted by @essential-randomness in #53 (comment)