Show/hide buttons on NavBar based on user role

[solebared]

Why

Required before we can re-enable user registration (#840).

What and How

• Move the logic of which buttons to show out of NavBar.vue and into a new headless policy NavBarPolicy.
• Custom method NavBarPolicy.visible_buttons returns an array of which buttons to show.
• This array is sent to NavBar.vue which guards each button by checking against the array.

Testing

• Added nav_bar_policy_spec.rb
• Corresponding modifications to NavBar.spec.js

# spec/policies/nav_bar_policy_spec.rb
NavBarPolicy
  visible_buttons
    guest
      is expected to contain exactly "Login"
      in peer_to_peer mode
        is expected to contain exactly "Contributions" and "Login"
    neighbor
      is expected to contain exactly "Feedback" and "Logout"
      in peer_to_peer mode
        is expected to contain exactly "Contributions", "Feedback", and "Logout"
    admin
      is expected to contain exactly "Contributions", "Matches", "Admin", "Feedback", and "Logout"
    sys_admin
      is expected to contain exactly "Contributions", "Matches", "Admin", "Feedback", and "Logout"


// spec/javascript/components/NavBar.spec.js
NavBar
  visible buttons
    when not instructed to show additional buttons
      ✓ shows publicly accessible links
    when instructed to show Contributions
      ✓ includes specified buttons
    when instructed to show all admin buttons
      ✓ includes specified buttons

Next Steps

• Bring this up to date with Admin param #849 once that is ready

Outstanding Questions, Concerns and Other Notes

?

Pre-Merge Checklist

• All new features have been described in the pull request
• Security & accessibility have been considered
• High quality tests have been added, or an explanation has been given why the features cannot be tested
• New features have been documented, and the code is understandable and well commented
• Entry added to CHANGELOG.md if appropriate
• All outstanding questions and concerns have been resolved
• Any next steps that seem like good ideas have been created as issues for future discussion & implementation