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

https://jira.duraspace.org/browse/DS-1622

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:
- Turn the feature on/off in dspace.cfg (jspui.user.assumelogin vs xmlui.user.assumelogin)
- Only super administrator can login as someone else (not himself)
- Can't log in as 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 built on top of the PR282 (reset password feature)