Split admin API for reported events into a detail and a list view (#8539)

Split admin API for reported events in detail und list view.
API was introduced with #8217 in synapse v.1.21.0.

It makes the list (`GET /_synapse/admin/v1/event_reports`) less complex and provides a better overview.
The details can be queried with: `GET /_synapse/admin/v1/event_reports/<report_id>`.
It is similar to room and users API.

It is a kind of regression in `GET /_synapse/admin/v1/event_reports`.  `event_json` was removed. But the api was introduced one version before and it is an admin API (not under spec).

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>