Don't check for allow_contact_requests in RosterItemView render method

This method shouldn't be called when contact requests aren't allowed because
roster items are then not created for contact requests.
A bit of reordering.

Move all module-level functions/variables and Backbone Modules/Views together.
Add headings to indicate logical sections.
Make statuses variable module-level and indicate that it's a constant (all caps)
New config option ``show_only_online_users``

If true, only users with a status of 'online' will be shown in the contacts
roster.