Default PowerLevelContent.Invite to 0 per matrix spec (#416)

The default power level for invite was wrong in matrix spec v1.1[1], and
has been fixed in v1.3, bringing us in line with synase[2].

[1]
matrix-org/matrix-spec@136b5c9
[2]
https://github.com/matrix-org/synapse/blob/v1.92.3/synapse/handlers/event_auth.py#L138

### Pull Request Checklist

* [x] Pull request includes a [sign
off](https://github.com/matrix-org/dendrite/blob/master/docs/CONTRIBUTING.md#sign-off)

Signed-off-by: `Nick Cao <[email protected]>`

Signed-off-by: Nick Cao <[email protected]>
Co-authored-by: Till <[email protected]>

Author: NickCao

event_test.go

@@ -169,7 +169,7 @@ func TestEventHistoryVisibility(t *testing.T) {
 }
 
 func TestEventPowerLevels(t *testing.T) {
-	eventJSON := `{"auth_events":[["$BqcTUuCsN3g6Rj1z:localhost",{"sha256":"QHTrdwE/XVTmAWlxFwHPW7fp3JioRu6OBBRs+FI/at8"}],["$9fmIxbx4IX8w1JVo:localhost",{"sha256":"gee+f1VoNeYGGczs5lwnUO1qeKAh70Hw23ws+YfDYGY"}]],"content":{"ban":50,"events":null,"events_default":0,"invite":50,"kick":50,"redact":50,"state_default":50,"users":null,"users_default":0,"notifications":{"room":50}},"depth":4,"event_id":"$1570trwyGMovM5uU:localhost","hashes":{"sha256":"QvWo2OZufVTMUkPcYQinGVeeHEODWY6RUMaHRxdT31Y"},"origin":"localhost","origin_server_ts":0,"prev_events":[["$QAhQsLNIMdumtpOi:localhost",{"sha256":"RqoKwu8u8qL+wDoka23xvd7t9UoOXLRQse/bK3o9qLE"}]],"prev_state":[],"room_id":"!roomid:localhost","sender":"@userid:localhost","signatures":{"localhost":{"ed25519:auto":"0oPZsvPkbNNVwRrLAP+fEyxFRAIUh0Zn7NPH3LybNC8lMz0GyPtN1bKlTVQYMwZBTXCV795s+CEgoIX+M5gkAQ"}},"state_key":"","type":"m.room.power_levels"}`
+	eventJSON := `{"auth_events":[["$BqcTUuCsN3g6Rj1z:localhost",{"sha256":"QHTrdwE/XVTmAWlxFwHPW7fp3JioRu6OBBRs+FI/at8"}],["$9fmIxbx4IX8w1JVo:localhost",{"sha256":"gee+f1VoNeYGGczs5lwnUO1qeKAh70Hw23ws+YfDYGY"}]],"content":{"ban":50,"events":null,"events_default":0,"invite":0,"kick":50,"redact":50,"state_default":50,"users":null,"users_default":0,"notifications":{"room":50}},"depth":4,"event_id":"$1570trwyGMovM5uU:localhost","hashes":{"sha256":"QvWo2OZufVTMUkPcYQinGVeeHEODWY6RUMaHRxdT31Y"},"origin":"localhost","origin_server_ts":0,"prev_events":[["$QAhQsLNIMdumtpOi:localhost",{"sha256":"RqoKwu8u8qL+wDoka23xvd7t9UoOXLRQse/bK3o9qLE"}]],"prev_state":[],"room_id":"!roomid:localhost","sender":"@userid:localhost","signatures":{"localhost":{"ed25519:auto":"0oPZsvPkbNNVwRrLAP+fEyxFRAIUh0Zn7NPH3LybNC8lMz0GyPtN1bKlTVQYMwZBTXCV795s+CEgoIX+M5gkAQ"}},"state_key":"","type":"m.room.power_levels"}`
 	event, err := newEventFromTrustedJSONV1([]byte(eventJSON), false, MustGetRoomVersion(RoomVersionV1))
 	if err != nil {
 		t.Fatal(err)

eventcontent.go

@@ -429,9 +429,9 @@ func NewPowerLevelContentFromAuthEvents(authEvents AuthEventProvider, creatorUse
 }
 
 // Defaults sets the power levels to their default values.
-// See https://spec.matrix.org/v1.1/client-server-api/#mroompower_levels for defaults.
+// See https://spec.matrix.org/v1.8/client-server-api/#mroompower_levels for defaults.
 func (c *PowerLevelContent) Defaults() {
-	c.Invite = 50
+	c.Invite = 0
 	c.Ban = 50
 	c.Kick = 50
 	c.Redact = 50