* Fix student list in teacher's dashboard

Russian constants are moved to twig & RusLang modules.
Functions were renamed to Semesters::load($id) & Semesters::loadAll().

It may seem that this commit is rather big, but it's not. Don't be afraid, just read the message. Recently we have understood that it was a bad idea to store rights for pages in a database. So I decided to move the information to PHP layer and make it more clear.

When you write a new controller in Admin/ Dean/ Student/ folder, you should explicitly extend one of Controller_Environment_* classes, and all specific checks will be done there. Additional advantage is manipulating $user variable, so you will have access to extended fields of user, ie User_Student contains StudentID, but User_Teacher does not.

Also, prefer -> operator to []. The reason is simple: IDE shows hints and mismatches only with the first one.

The second important statement is User::checkAccess(User::RIGHTS_*) — call it at the top of *every* function in the Handler/ classes. Exception will be thrown, if user doesn't have enough rights.

NB. If you want to overload "before()" function while extending Environment_* controller, do it in the right way:

    public function before() {
        parent::before();   // always first

        // any magic you like most
    }

* Controller_UserEnvironment is an abstract class, which defines $twig variable, and calls $twig->render in the after() method.
* $twig contains $twig->User variable by default.
* As I have read, it's totally recommended not to use $twig->Variables, so I defined $twig->set([params]) syntax
* Twig supports "fluent interface", so use $twig->set(...)->set_filename();
* Divided DeanOffice/Credits.php & moved part to Handler/

+ new API methods
+ some code refactoring

1. Discipline models have the direct access to fields, for example: $discipline->subjectName, so you don't have to remember them (press ctrl+space). Improves static code analysis. Also, if you make a mismatch, ErrorException will be thrown — this is the most important improvement.

2. There are some operations on disciplines (attach new teacher, transfer discipline to another teacher, an so on), so you can use them: $discipline->attachTeacher(53).

3. Discipline model has array access operators, so you must not convert instances with toArray() method. A common way is:

/* PHP */
$d = Model_Discipline::load($id);
$d['extraField'] = 127;
$twig->Discipline = $d;  // is ok, cause $d is like an array

/* Twig */
{{ Discipline.extraField }}   // call $d->offsetGet('extraField') method

4. Models Faculty, Student, Teachers and Groups have `getDisciplines()` method, and an array of Model_Discipline will be returned.

5. We use camelCase for all fields: in twig, on the PHP level and also in the database.

Use $user or User::instance() instead

Reduced usages of UserInfo

That's why a conflict occurs when a class has two functions with the same name.

2. Исправление верстки на странице редактирования персональных данных