MDL-28452 core_user: migrate social profile fields

MDL-28452 core_user: migrate social profile fields

Create a new profile field type, move all existing content of the fields
'icq', 'skype', 'aim', 'yahoo', 'msn' and 'url' in the mdl_user table to
theses new profile fields if needed.

AMOS BEGIN
MOV [aimid,core],[aimid,courseimage,profilefield_social]
MOV [yahooid,core],[yahooid,profilefield_social]
MOV [skypeid,core],[skypeid,profilefield_social]
MOV [icqnumber,core],[icqnumber,profilefield_social]
MOV [msnid,core],[msnid,profilefield_social]
MOV [webpage,core],[webpage,profilefield_social]
AMOS END