Implement GET channels endpoints for APIv4 (#5363)

* implement get channels endpoints and updated drivers and unittests

* removed channel deletion on tear down, removed manage permission on get channels endpoints, and updated utils to add constant channel length

* added constants for user, team and channel length, updated context to use the model functions

* make sure team name length should be less than the minimum length and revert underscore to team name validity

* changed post test condition from notfound to unauthorized

model/utils.go

@@ -280,7 +280,7 @@ func IsValidChannelIdentifier(s string) bool {
 		return false
 	}
 
-	if len(s) < 2 {
+	if len(s) < CHANNEL_NAME_MIN_LENGTH {
 		return false
 	}