Make "User ID" column customizable in the User Directory livetable

By default, in 5.0, the User Directory shows 'Picture', 'User ID', 'First Name' and 'Last Name' columns.

Since 4.0 ( XE-1131 ) the User Directory is customizable, but the columns 'Picture' and 'User ID' can't be removed.

Right now (see 5.0.png) the 'User ID' column displays something like 'xwiki:XWiki.Admin' which is technical information and not very user friendly.

We could:

• not display this 'User ID' column by default (and rely that the users will use the suggest XWIKI-4022 and they don't need to know the ID ) or
• move it after the: 'Picture', 'First Name' and 'Last Name' on the forth position in the livetable, giving this column a lover importance, while still displaying it.

Issue reported by Vincent Massol