User custom fields can now be set as editable so that users can edit them on 'My account'.

For existing user custom fields, this new attribute is set to false by default to preserve the prior behaviour (it can turned on by editing the custom field in admin area).

Note: on the registration form, *required* custom fields will be displayed even if they are not defined as editable so that the account can be created.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@2276 e93f8b46-1217-0410-a6f0-8f06a7374b81