For parsing `oauth2_UsernameField` values like
`profile.preferred_username`, this makes use of `eval()` which is
generally Evil™, but I'm assuming that anyone with permission to edit
config.json likely has permission to make changes to the fedwiki source
code already anyway, so it's fragile rather than increasing a security
attack surface. An alternative would be using a small function to look
up properties of the `params` / `profile` objects using the same
dotted-path notation.
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