Escape eventIds in CS API requests

[manuroe]

Event IDs in room version 3 can now include hashes in them according to MSC1659.

APIs that must be checked:

/admin/purge_history/(?P<room_id>[^/]*)(/(?P<event_id>[^/]+))?
/events/(?P<event_id>[^/]*)$
/rooms/(?P<room_id>[^/]*)/report/(?P<event_id>[^/]*)$
/rooms/(?P<room_id>[^/]*)/event/(?P<event_id>[^/]*)$
/rooms/(?P<room_id>[^/]*)/context/(?P<event_id>[^/]*)$
/rooms/(?P<room_id>[^/]*)/redact/(?P<event_id>[^/]*)
/rooms/(?P<room_id>[^/]*)/receipt/(?P<receipt_type>[^/]*)/(?P<event_id>[^/]*)$

[turt2live]

we should double-check these for riot-web as well