Cleanup js files

This pull request doesn't change anything in logic layer.

Changes:
 - added ``'use strict';`` on top of the files,
 - added semicolon at end of line,
 - removed unnecessary semicolon after ``if``, ``for``, ``switch``,
 - ``===`` instead of ``==``,
 - single quotes instead of double quotes,
 - ``.join('/')`` instead of concatenate urls with ``+`` .

Please review @mkurek @kula1922