Support unarchiving threads

Setting ChannelEdit.Archived = false is currently not possible since go
will treat a false as empty and will omit the property from the JSON
object.

structs.go

@@ -370,7 +370,7 @@ type ChannelEdit struct {
 
 	// NOTE: threads only
 
-	Archived            bool `json:"archived,omitempty"`
+	Archived            bool `json:"archived,bool"`
 	AutoArchiveDuration int  `json:"auto_archive_duration,omitempty"`
 	Locked              bool `json:"locked,bool"`
 	Invitable           bool `json:"invitable,omitempty"`