Fix login link on explorer

The login link is not always accurate (see preview): fix behaviour modified with NXP-29263

Steps to reproduce:

1. go to http://localhost:8080/nuxeo/site/distribution
2. click on "login" on top right corner: redirected to http://localhost:8080/nuxeo/login.jsp?forceAnonymousLogin=true&requestedUrl=/nuxeo/site/distribution
3. do not login and hit "back" and refresh the page
4. click on "login" again -> redirection goes to "http://localhost:8080/nuxeo//nuxeo/site/distribution" this time, with error "Sorry, the page you requested cannot be found."
5. any following attempt to get to the login page redirects to this invalid URL

Once in this state, to get back the login page, need to go to call http://localhost:8080/nuxeo/logout to get properly redirected to the login page.