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/__init__.py

@@ -24,7 +24,7 @@ login_manager.login_message = LOGIN_MANAGER_MESSAGE
 
 
 def get_ldap_handler(env):
-    if env == "deeev":
+    if env == "dev":
         return DummyLdapAuthenticator()
     return DocLdapAuthenticator()