Feature Requests Add API endpoint for querying downtimes across all checks

According to the documentation there is currently no API endpoint equivalent to the listing of downtimes found in the user panel (https://updown.io/downtimes).

At the moment, downtimes can only be retrieved pre-filtered by check.

It would be nice to have a paginated API endpoint providing a list of downtimes across all checks, e.g. /api/downtimes.