open edX: display user's full name instead of username in user menu

[pdpinch]

In mitxpro-theme, we override the header template to display the user's full name instead of their username. We'd like to do the same thing with the MFEs that rely on frontend-component-header-edx

We started to pursue this for mitxonline, but then stopped because the product requirements changed. See mitodl/mitxonline#132

Acceptance Criteria:

• fork frontend-component-header-edx into a repo named frontend-component-header-mitxonline
• update the user menu to display the user's full name, instead of their username
• work with DevOps to deploy the new component
• update the checklist if any additional steps are discovered

[pdpinch]

We should consider whether to do this, or to follow the route of mtixonline where users choose their own username.