refactor: add server-side validation schema for events (#4622)

* refactor: move files to subfolder

* refactor: schema for events.list

* refactor: update nullable fields in Event model

* fix: event actor not nullable

* fix: team not nullable

server/routes/api/__snapshots__/events.test.ts.snap

@@ -1,10 +0,0 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
-
-exports[`#events.list should require authentication 1`] = `
-Object {
-  "error": "authentication_required",
-  "message": "Authentication required",
-  "ok": false,
-  "status": 401,
-}
-`;