Allow users to customize their view of the user directory

The current user directory is very limited with respect to the information it displays and the amount of user search that can be performed based on various criteria.

The proposed improvement implies allowing each user to build a customized view of the user directory, where properties of the XWikiUsersClass can be added as columns. This goes hand in hand with the previous work done for the customization of the user profile based on the same properties of the XWikiUsersClass (XWIKI-7626).

Having these extra columns, a user can now search for other users by using the livetable's filter and sort functions. A reset to defaults option should also be available to quickly discard the customization, should the user choose to.