Refactor Custom project templates page to Vue

Summary

The custom project templates page is currently using a combination of HAML partials with custom JavaScript to handle tabbing and pagination. The current implementation is brittle (already caused two recent regressions) and difficult to maintain.

I suggest we refactor this page to use Vue components for the tabs and list.

Improvements

Refactoring this view to Vue.js will help prevent future regressions and make it easier to maintain.

Involved components

TBA

Optional: Missing test coverage

I sugggest we add integration tests for this page if we don't have it by the time we pick up this issue.