MM-42535: Set ThreadAutoFollow to true by default (#19797)

Summary
A number of performance improvements shipped in v6.5 to support CRT with the ThreadAutoFollow bookkeeping enabled but the frontend features disabled. To prepare for a later enabling of the frontend features by default, we want to encourage customers to re-enable ThreadAutoFollow, and plan to enable this by default in v6.6.

Ticket Link
https://mattermost.atlassian.net/browse/MM-42535

app/channel_test.go

@@ -2038,6 +2038,9 @@ func TestMarkChannelsAsViewedPanic(t *testing.T) {
 	mockPreferenceStore.On("Get", mock.AnythingOfType("string"), mock.AnythingOfType("string"), mock.AnythingOfType("string")).Return(&model.Preference{Value: "test"}, nil)
 	mockStore.On("Channel").Return(&mockChannelStore)
 	mockStore.On("Preference").Return(&mockPreferenceStore)
+	mockThreadStore := mocks.ThreadStore{}
+	mockThreadStore.On("MarkAllAsReadByChannels", "userID", []string{"channelID"}).Return(nil)
+	mockStore.On("Thread").Return(&mockThreadStore)
 
 	_, appErr := th.App.MarkChannelsAsViewed([]string{"channelID"}, "userID", th.Context.Session().Id, false)
 	require.Nil(t, appErr)

model/config.go

@@ -781,7 +781,7 @@ func (s *ServiceSettings) SetDefaults(isUpdate bool) {
 	}
 
 	if s.ThreadAutoFollow == nil {
-		s.ThreadAutoFollow = NewBool(false)
+		s.ThreadAutoFollow = NewBool(true)
 	}
 
 	if s.CollapsedThreads == nil {