PLT-6217 Fixes system message posted to Town Square when a member leaves a team (#7752)

* Fixes system message posted to Town Square when adding/removing team member (#6483)

Fixes unit test method call to match RemoveUserFromTeam modification (#6483)

Fixes system message posted to Town Square when adding/removing team member (#6483)

Removes unnessary error message

* Updates system message when a user leaves a team (#6483)

* Changes system message when a user is removed from team (#6483)

* Adds a new string to be post to town-square when a member is added to a team (#6483)

model/post.go

@@ -19,8 +19,10 @@ const (
 	POST_JOIN_LEAVE            = "system_join_leave" // Deprecated, use POST_JOIN_CHANNEL or POST_LEAVE_CHANNEL instead
 	POST_JOIN_CHANNEL          = "system_join_channel"
 	POST_LEAVE_CHANNEL         = "system_leave_channel"
+	POST_LEAVE_TEAM            = "system_leave_team"
 	POST_ADD_REMOVE            = "system_add_remove" // Deprecated, use POST_ADD_TO_CHANNEL or POST_REMOVE_FROM_CHANNEL instead
 	POST_ADD_TO_CHANNEL        = "system_add_to_channel"
+	POST_ADD_TO_TEAM           = "system_add_to_team"
 	POST_REMOVE_FROM_CHANNEL   = "system_remove_from_channel"
 	POST_HEADER_CHANGE         = "system_header_change"
 	POST_DISPLAYNAME_CHANGE    = "system_displayname_change"
@@ -167,8 +169,8 @@ func (o *Post) IsValid() *AppError {
 	}
 
 	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE ||
-		o.Type == POST_JOIN_CHANNEL || o.Type == POST_LEAVE_CHANNEL ||
-		o.Type == POST_REMOVE_FROM_CHANNEL || o.Type == POST_ADD_TO_CHANNEL ||
+		o.Type == POST_JOIN_CHANNEL || o.Type == POST_LEAVE_CHANNEL || o.Type == POST_LEAVE_TEAM ||
+		o.Type == POST_REMOVE_FROM_CHANNEL || o.Type == POST_ADD_TO_CHANNEL || o.Type == POST_ADD_TO_TEAM ||
 		o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE || o.Type == POST_PURPOSE_CHANGE ||
 		o.Type == POST_DISPLAYNAME_CHANGE || o.Type == POST_CHANNEL_DELETED ||
 		strings.HasPrefix(o.Type, POST_CUSTOM_TYPE_PREFIX)) {