Default Channel Functionality + Test Suite (#9068)

gofmt

Make skipping for non public channels default

Deduplication of Default Channels; Only post join to townsquare

Post join channel message for all custom default channels

model/config.go

@@ -1105,6 +1105,7 @@ type TeamSettings struct {
 	ExperimentalHideTownSquareinLHS     *bool
 	ExperimentalTownSquareIsReadOnly    *bool
 	ExperimentalPrimaryTeam             *string
+	ExperimentalDefaultChannels         []string
 }
 
 func (s *TeamSettings) SetDefaults() {
@@ -1217,6 +1218,10 @@ func (s *TeamSettings) SetDefaults() {
 		s.ExperimentalPrimaryTeam = NewString("")
 	}
 
+	if s.ExperimentalDefaultChannels == nil {
+		s.ExperimentalDefaultChannels = []string{}
+	}
+
 	if s.EnableTeamCreation == nil {
 		s.EnableTeamCreation = NewBool(true)
 	}