Added authentication to metro dashboard

[antidipyramid]

Overview

This PR adds authentication to the LA Metro Dashboard plugin page. Users will be required to login to view it.

After this change is merged in, we will create login accounts for our colleagues at Metro.

Checklist

• [x] PR has a descriptive enough title to be useful in changelogs

Testing Instructions

• Navigate to the dashboard and verify that the page is inaccessible if not logged in

Handles #10

[antidipyramid]

@hancush Sure, I can add a note to the README.

Perhaps the way forward would be to create a new Role (e.g. 'Metro Admin') that would have custom permissions. Giving that role the permission can list on Dashboard seems give access to only the Dashboard page. Testing locally, it does work but the redirect from the login page would need to be tweaked.

[hancush]

@antidipyramid Ok, awesome, love that idea. I think it's probably fine to manually create that permission (rather than adding it in a fixture), but let's document that here, as well, and make any code change necessary to handle that redirect. Does that work?