Replace user_select in app/assets/javascripts/pages/projects/boards/index.js

We're in the process of migrating away all uses of Bootstrap. Whilst this is a simple task for things such as buttons and alerts, dropdowns are much more complex. Because of that, there will be no step-by-step instructions in this issue.

This dropdown can (very likely) be migrated to our user_select Vue component, for more info see the epic: &8531