Fix the default image url passed to gavatar. Bug 1378543

This url must not include a querystring
(see https://en.gravatar.com/site/implement/images/)

- Return the no user photo url if the remote avatar is not found
- Try to use profile_icon_url as much as possible

Change-Id: Id18ee4469b1bd15013445f8f4dc4a582dee659e8
Signed-off-by: Son Nguyen <sonn@catalyst.net.nz>