Review built-in help system, add links to customizable Help buttons

[prioux]

Some of our pages have a Help button that's not so useful, and could probably be removed. Found in: AccessProfiles index, Bourreaux index, Feedbacks index, Groups index, Tools index, Messages index, Sites index, Userfiles index (dead documents!)

How would these be replaced?

Well, all our pages should be adjusted to have the customizable Help button that Rémi created last summer. The only place where the feature is deployed right now is in the DataProviders index page. This allows the admin to add notes and information for all users in neat markdown language; the info is saved in a file in the "public/doc" area of the server. Even better, within the CBRAIN distribution we could provide pre-filled versions of these files, and as long as we make sure they are somehow registered in the mini database of the HelpDocument model, it would serve as a replacement for the other Help buttons we removed (as described in my first paragraph here). Rémi provided a method from_existing_file!() that could be used to bootstrap the pre-load of the help documentation, maybe as a rake task or a data migration.