Replace the livetable of the User Directory with a Live Data

There are actually 2 LT to convert:

• The one displayed on Main.UserDirectory
• The one displayed in the Admin UI to configure the user directory columns

However both reuse the same code from ##XWiki.UserDirectoryMacros##, so we need to convert both at the same time.

Also note that the LT uses the Tag Cloud feature which currently doesn't exist in LDs (see XWIKI-19037).

Start of some implementation done in https://github.com/xwiki/xwiki-platform/commit/0c8d6e760fe1954d096729c6c80683b5420b97ff