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

api4/user_test.go

@@ -154,7 +154,7 @@ func TestGetUserByUsername(t *testing.T) {
 	_, resp = Client.GetUserByUsername(GenerateTestUsername(), "")
 	CheckNotFoundStatus(t, resp)
 
-	_, resp = Client.GetUserByUsername(model.NewRandomString(25), "")
+	_, resp = Client.GetUserByUsername(model.NewRandomString(1), "")
 	CheckBadRequestStatus(t, resp)
 
 	// Check against privacy config settings