After login/logout, can not redirect to where user comes from.

[Jiang Xin]

Can not redirect to the orignal place where user comes from, if login or logout using TracCoSignPlugin.

For example:

• user visit ​http://trac-hacks.org/report/9?COMPONENT=TracCoSignPlugin without login;
• user click login link in navibar (TracCoSignPlugin installed);
• redirect to CoSign single sign-on for login;
• redirect back to ​http://trac-hacks.org/wiki;

But, it should redirect back to ​http://trac-hacks.org/report/9?COMPONENT=TracCoSignPlugin

[Jiang Xin]

(In [5226]) TracCoSignPlugin: Take referer as query_string between trac and cosign sso login/logout process; This hopefully fixes #4600.