Allow to avoid typing http / https when connecting to a Nuxeo server

Add a utility method that computes all URLs we want to try after the user has entered a server URL:

• add http / https before if not present
• append /nuxeo at the end of the URL

For example, typing demo.nuxeo.com, we should end up with the following list of URLs:

• http://demo.nuxeo.com
• http://demo.nuxeo.com/nuxeo
• https://demo.nuxeo.com
• https://demo.nuxeo.com/nuxeo

Typing https://demo.nuxeo.com should result of:

• https://demo.nuxeo.com
• https://demo.nuxeo.com/nuxeo

To check a URL, do a fetch request with the following parameters to avoid any redirect:

{ method: 'HEAD', redirect: 'manual'}

To do for both acquire_token.jsp and basic authentication.

An optimization could be to check the redirect location of the response to use it as next URL to test.