Fix documentation

Fix typos and improve documentation for "setup_group_settings_tabs",
"setup_operator_settings_tabs" and "setup_settings_tabs" functions.
Do not use global $page array in setup_statistics_tabs func
Add second argument to \Mibew\Style\StyleInterface::render
Do not use global $page variable in form_value func
Do not use global $page variable in form_value_cb func
Do not use global $page variable in form_value_mb func
Replace expand* function with TemplateEngine class
Fix bug with close button in chat window
Fix bug with send chat history to email
Remove global variable $session_prefix

Use constant SESSION_PREFIX instead of global variable
Remove $default_cron_key global variable

Use DEFAULT_CRON_KEY constant instead of global variable
Use different pagination images for different styles
Rewrite pagination functions

Remove global variable $page from pagination functions, refactor
them and add inline documentation.
Remove global variable $page from prepare_menu func
Remove global variable $page from set_form_date func
Remove global variable $page from setup_redirect_links func
Do not use global variable $errors in verifyparam_groupid func
Do not use global variable $dbversion

Replace $dbversion global variable with DB_VERSION constant
Do not use $featuresversion global variable

Replace $featuresversion global var with FEATURES_VERSION constant
Do not use $version global variable

Replace $version global var with MIBEW_VERSION constant