Change Trac /login link to point to /trac/github/login

[rodrigc]

If I am not logged in, and click on:

https://twistedmatrix.com/trac/newticket

It takes me to a page:

 You are currently not logged in. You may want to do so now.

If I click on do so, it takes me to:

No handler matched request to /login

Can this be fixed on the Trac side so that /login redirects to:
https://twistedmatrix.com/trac/github/login ?
Or change do so to hyperlink to /trac/github/login ?

This is a stumbling block when new contributors are asked to open a ticket in Trac.