Since we may not want to enable the identity check, it must be
togglable. This commit adds environment variable
REACT_APP_ENABLE_IGA_CHECK for toggling the check in the frontend, and
django settings variable IGA_CHECK for toggling the check in the
backend.

Queries iga for an external id matching ours. If a match is found, the
name of the owner is returend. Otherwise we return null. Uses the new
iga module to fetch the data.