support impersonating other users.

Support for impersonating other users using an impersonation operator when specifying the username.

The current impersonation operator is as. The left-hand argument is the actual user logging in, the right-hand argument is the impersonated user.

Example of how to use the operator:

app/ldap_authentication/authenticator.py

@@ -80,9 +80,8 @@ class DocLdapAuthenticator(Authenticator):
         users = re.match(IMPERSONATION_PATTERN, username)
         if users:
             impersonator, impersonated = users.groups()
-            if impersonator in IMPERSONATORS:
-                username = impersonator
-                logging_in_as = impersonated
+            username = impersonator
+            logging_in_as = impersonated if impersonator in IMPERSONATORS else impersonator
         connection = ldap.initialize(self.server_url)
         connection.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_ALLOW)
         connection.set_option(ldap.OPT_X_TLS_NEWCTX, 0)