ikiwiki hosting internals/ todo/ some SSL improvements

Please consider my `ready/ssl` branch: http://git.pseudorandom.co.uk/smcv/ikiwiki-hosting.git/shortlog/refs/heads/ready/ssl

Changes are:

- Include /etc/ikiwiki-hosting/b-foo/apache-ssl.conf.tmpl in the SSL vhost, but not the unencrypted vhost. This is a good place to configure HTTP basic/digest authentication or adjust SSL ciphers, for instance.

- Similarly, for completeness, include /etc/ikiwiki-hosting/b-foo/apache-source.conf.tmpl in the source.foo vhost.

- apache.conf.tmpl is included in all vhosts (unencrypted, SSL and source) as before.

- Give sites a boolean redirect_to_https option. If on, the normal port-80 vhost behaves like the aliases, redirecting to the SSL vhost.

- If a SSL key exists, but redirect_to_https is not set, unencrypted aliases redirect to the unencrypted main site (but SSL aliases redirect to the SSL site).

That last change makes it much more palatable to have a public, mostly-read-only site that doesn't need SSL for normal use, but install a self-signed or otherwise non-cartel-approved certificate so that a few authorized editors (who can be taught to verify the self-signed cert by fingerprint) can use password authentication securely. For instance, that's probably what I'm going to do for my blog.

Truth table: suppose www.example.com is the canonical/preferred name of example.com, aka example.branchable.com.

    redirect from  |r_t_https=0  |r_t_https=1  | old behaviour
    ---------------+-------------+-------------+-------------
    http://www.e.c |(no redirect)|https://www  |(no redirect)
    https://www.e.c|(no redirect)|(no redirect)|(no redirect)
    http://e.c     |http://www   |https://www  |https://www
    https://e.c    |https://www  |https://www  |https://www
    http://e.b.c   |http://www   |https://www  |https://www
    https://e.b.c  |https://www  |https://www  |https://www

--smcv

> merged --Joey