Start, Badge, and Task Chain filters for visitors

• Fix #147: Prevent visitors from bypassing filters
  • Add option for hiding filtered task content from visitors (logged-out users), checked by default (this is retrospective)
  • Add option for hiding task content from visitors regardless of filters
  • Prevent visitors from accessing content in the following order:
    1. User only, stop
    2. Start filter, if the start date hasn’t passed
    3. Task Chain filter, if there is a prerequisite task
    4. Badge filter, if there are any valid badges required
  • Normal users will be prevented from accessing content based on b-d from above
  • Display task pagination for visitors, when there are no filters in the way
  • Display a login link in the Start, Badge, and Chain filters errors, for convenience
• Fix #142: consistent styling for Start filter and Task Chain filter errors
  • Apply similar styles to the Start, Badge, and Chain filters