Use a gopher as default avatar (rather than the gravatar logo) (#3208)

Also changes the avatar from a jpeg to a png, to allow for
transparent background. The indexed png is also smaller in size.

Note that at the moment the default avatar is only used when
the user requested a custom avatar and the custom avatar file
is not found (should never happen).

In the future the default avatar could be used as a default
return when by-mail avatar lookups fail too (both gravatar
and libravatar support passing a default)

modules/base/tool.go

@@ -207,7 +207,7 @@ func HashEmail(email string) string {
 // AvatarLink returns avatar link by given email.
 func AvatarLink(email string) string {
 	if setting.DisableGravatar || setting.OfflineMode {
-		return setting.AppSubUrl + "/img/avatar_default.jpg"
+		return setting.AppSubUrl + "/img/avatar_default.png"
 	}
 
 	return setting.GravatarSource + HashEmail(email)