Add the ability to set the default language for new users.
Admin can set server default language in admin/General section.

Setting allows admin to modify Meteor.loginTokenExpires from default 90 days to something else.
Server restart is required for this setting to take effect.
Default value is still 90 days if not modified by admin.

The avatar image move need to only be done if a previous username exists. 
Also we need `encodeURIComponent` to support names in unicode.