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

@@ -79,8 +79,7 @@ class DocLdapAuthenticator(Authenticator):
         logging_in_as = username
         users = re.match(IMPERSONATION_PATTERN, username)
         if users:
-            impersonator = users.groups()[0]
-            impersonated = users.groups()[1]
+            impersonator, impersonated = users.groups()
             if impersonator in IMPERSONATORS:
                 username = impersonator
                 logging_in_as = impersonated