Fix for a few strict standards warnings

This pull request fixes two different warnings appearing in strict standards mode:

The first one has to do with an imlementation of `Smarty::assign` which has a missing argument:
```
Strict standards: Declaration of SmartyHackademic::assign() should be compatible with
Smarty_Internal_Data::assign($tpl_var, $value = NULL, $nocache = false) in
model\common\class.SmartyHackademic.php on line 38
```

The second appears because a few methods from `HackademicController` are called in a static way but are declared non-static. I made the methods static (They were essentially calls to `Session::isXXXX`).
```
Strict standards: Non-static method ChallengeMenuController::go() should not be called statically,
assuming $this from incompatible context in controller\class.HackademicController.php on line 107
```

I also found a few controller classes which don't inherit from `HackademicController` (such as `ChallengeMenuController` or `UserMenuController`). If that's an error, I can correct it in this pull request before merging.