Add code to have authopenid handle /login

[puiterwijk]

This code makes it possible to have authopenid take /login.
This is useful because all trac's "not authorized" messages are hardcoded to point to /login, and if openid is the only allowed provider, this leaves users with a 404.

[dairiki]

I will look into this further in a couple of weeks, when I return from vacation.

In the mean time, note the that hardcoded urls to the /login page can be suitably mangled by including something like:

<a py:match="a[@href='/env/login']" href="/env/openidlogin">
  ${select("*|text()")}
</a>

in your env/templates/site.html.