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/post_test.go

@@ -159,7 +159,7 @@ func TestGetPostsForChannel(t *testing.T) {
 	}
 
 	_, resp = Client.GetPostsForChannel("", 0, 60, "")
-	CheckNotFoundStatus(t, resp)
+	CheckUnauthorizedStatus(t, resp)
 
 	_, resp = Client.GetPostsForChannel("junk", 0, 60, "")
 	CheckBadRequestStatus(t, resp)