Based on #3340
This adds a `/_synapse/admin/v1/event_reports` endpoint, the same
Synapse has. This way existing tools also work with Dendrite.
Given this is already getting huge (even though many test lines),
splitting this into two PRs. (The next adds "getting one report" and
"deleting reports")
[skip ci]
Part of #3216 and #3226
There will be a follow up PR which is going to add the same admin
endpoints Synapse has, so existing tools also work for Dendrite.