Add enable_per_project_billing field to companies.
Enable billing Checkbox on company edit.
Only show billing related things if enabled at the company level

  Affected areas:
  * Billing tab in the top menu (+ restrict access to billing)
  * Services under Admin panel
  * Service under Task atttributes
  * Suppress Billing under Project show and edit
Adjust billable logic.
Hungarian localization.
Rename company enable_per_project_billing attribute to use_billing (migration change also!).
Added spec for User#can_use_billing?