[DS-1622] Porting of the Login As feature to JSPUI 

Porting of the administrative loginAs feature to the JSPUI. The code is not a copy and paste from XMLUI for the different behaviour to manage the same attribute in the session, but it mantains the same rules:
- Add in dspace.cfg to on/off the feature (jspui.user.assumelogin vs xmlui.user.assumelogin)
- Only super administrators can login as someone else (not himself)
- No login of another super administrator
- Set any special groups - invoke the authentication mgr.

The authentication logic is does into loggedIn method on JSPUI Authenticate.java class.

Take care that this code is build on top of the PR282 (reset password feature)