Added variable authelia.redirect_service

For #446 .

I added a variable to persist Authelia redirection. As the variable is stored in the vault, due to the fact that the authelias service is described there, any existing vault.xml file would need to be upgraded??

I need help on doing that, if it is needed. I am not sure if it is.

Tested the solution with ./set_setting authelia.redirect_service and determined it was propagated to vault.xml and into the authelia config Yaml file.