ikiwiki hosting internals/ todo/ directory

Wanted: Directory of sites. Could be used for:

- internal monitoring of what sites are being made and used
- possibly, a public directory (modulo privacy concerns)

Data structure could be a directory with pages, created by ikisite when it creates sites, by filling out a template. (And removed by ikisite when it deletes sites.) Then the pages could be used to build a rss feed of sites, or they could have aggregation directives, to make a planet of their recentchanges feeds.

So, it needs to support multiple observing sites. At the simplest, it could be configured thus:

    # List of sites that will be updated when sites are added/removed.
    # These sites should each have a template named observesite.tmpl, which
    # will be filled out to create pages under sites/existing/ and
    # sites/removed/
    observersites="internal.branchable.com www.branchable.com"

`ikisite observe hostname` can be called with either an existing, or a non-existing hostname, and will update its pages for each of the observed sites. It should only create the page for a hostname if it does not already exist (to allow us to annotate hostnames with our comments, etc). When removing a page, it should move it to removed/ (preserving its content). It should also move any subpages associated with the site. That way we can add comments, screenshots, etc.

If we had a privacy flag, sites would need to be hidden if the flag was set, and unhidden if it was cleared. Deferred for later. We'd probably want to manually review sites in a public directory anyway, and tag ones we want to publish.

done