Enable configuration of $obfuscateEmail variable in user sheet

Currently, $obfuscateEmail variable is hard coded in page XWiki.XWikiUserProfileSheet. We should set a default value after we have checked that it is not yet set. I propose to update that page from :

#set($obfuscateEmail = true)

to

#if("$!{obfuscateEmail}" == '')
  #set($obfuscateEmail = true)
#end

Tested locally on a 3.0-M3 with this change and updating xwikivars.vm, works as expected.