global: fix standard flash messages security

• SECURITY Fixes potential XSS issues by changing main flash messages template so that they are not displayed as safe HTML by default.

• NOTE Displaying HTML safe flash messages can be done by using one of these flash contexts: '(html_safe)', 'info(html_safe)', 'danger(html_safe)', 'error(html_safe)', 'warning(html_safe)', 'success(html_safe)' instead of the standard ones (which are the same without '(html safe)' at the end).

Signed-off-by: Nicolas Harraudeau <nicolas.harraudeau@cern.ch>