Make `user` field read-only in `ExtraInfo` detail
Make `user` field read-only in `ExtraInfo` detail

Previously, server would hang trying to load an `ExtraInfo` detail page,
because the `user` field was rendering as a dropdown select menu loading
all users in the system. We fix this by making the field read-only.

Stanford-Online/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform