Work Items: Allow users to select which attributes/metadata to show in different views

Summary

Work Items include support for various additional attributes like Assignees, Labels, Start/Due Dates, Milestones, and etc. Also, all these attributes also show up in various places like Hierarchy Widget for instance, this may end up making the UI busier, not to mention that users may not be interested in seeing all the attributes all the time.

In some places, we do allow users to toggle Labels visibility (eg; Issue Boards), but still there are other attribute types which may not be very useful to users in certain places.

Proposal

We can allow user to have control over which attributes are visible in which places for certain Work Item types. We already have required frontend components to build this interface so it is just a matter of building the backend and determining where on UI we should place these controls, along with user permissions needed to alter this.