Adding a custom X509 username field.
Review Request #1554 — Created Feb. 11, 2019 and submitted
This will allow for configurations that use other front-ends for authentication. One example might be mod_auth_mellon for SAML SSO support (which sets MELLON_NAME_ID). Testing Done: Not sure how to inject `settings` in `test_x509_auth_middleware tests` to test "CUSTOM". Tested on a copy of our reviewboard server which uses mod_auth_mellon and references our SSO server. Works like a charm! Reviewed at https://reviews.reviewboard.org/r/10131/