add helpdesk links

Support requests should be made through rt. In production we have two links (because we have two rt submission forms, but which lands into the same queue) it is configurable at runtime in the helpdesk_link config key

# from rails/config/application.rb
# Externalised web links inside the help dropdown bar
config.helpdesk_links = {
   "report a problem" => "https://helpdesk.inria.fr/categories/227/submit",
   "ask a question"   => "https://helpdesk.inria.fr/categories/228/submit",
}

This should be configurable at runtime, in our deployment we could have:

{
    "Helpdesk":             "https://helpdesk.inria.fr/categories/227/submit",
    "Bug tracker (public)": "https://gitlab.inria.fr/allgo/allgo.inria.fr/issues", 
    "Contact":              "mailto:allgo@inria.fr",
}

The "ask a question" (demande pour allgo) form is kind of overlaping with allgo.inria.fr/issues, maybe we can drop it