Use the space key instead of the space name when possible

In some versions the spaces can have both a name and a key in which case some URL and some elements may refer to the key instead of the name.

We need to create an index of the spaces based on the key in a first pass so that links have the right space name.

Also in Confluence world the key is supposed to be the one used in the URL so we should probably use the key instead of the name when generating reference and space event (since in practice XWiki only have a key, it's just that it support anything as a key). Would also make the first part of this issue easier (no need for an index anymore).