Generate valid CSS/class name for custom status with spaces

Prior to this, for a custom status 'in progress', Mantis would generate
the following invalid code:

• HTML (class name): class="small-caption in progress-color"
• Dynamic CSS (status_config.php): .in progress-color { ... }

We now rely on a new API function html_get_css_identifier() to generate
a valid CSS class name by replacing invalid characters with '-'.

Fixes 0020068