Expose logged-in LMS user basic details to the FE via config

[acelaya]

A requirement for the LMS dashboard is that we display the logged-in user display name in the header.

This PR adds a new config entry which exposes the most basic user details for the FE to display. It currently includes only the display_name.

This information is not used yet. It will be used as part of https://github.com/hypothesis/lms/pull/6372

Testing steps.

1. Launch an assignment
2. Open the browser console, and inspect generated HTML.
3. In the LMS iframe, we generate a <script />qq tag withclass="js-config"`. It should include a block like this:

   {
     "user": {
       "display_name": "Hypothesis 101 Teacher"
     }
   }