Use the main wiki descriptor server field for the domain name even in path based mode

Right now the server field in the wiki descriptor has a different meaning depending if we are in path based or domain based mode:

• domain based: the domain name of the URL
• path based: the alias used to indicate the target wiki in the path

This means there is only two way to know the domain name for external URL in path based mode:

• the URL of the first request
• the property xwiki.home in xwiki.cfg file

This also means there is no way to indicate the domain at wiki level contrary to domain based for which you can use the descriptor.

Since the alias is never used for the main wiki I propose (initially suggested by @gdelhumeau) to change the meaning of the server field in the main wiki to indicate the domain to use when generating the external URLs no matter the mode.

See the discussion at https://forum.xwiki.org/t/use-the-main-wiki-descriptor-server-field-for-the-domain-name-even-in-path-based-mode/5629