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