Adds wiki to each Site. The wiki index is accessible through SITE-BASE-URL/wiki, each page is SITE-BASE-URL/wiki/PAGENAME -same as the regular project wikis.

The wiki is a simple Grit object, not a full-on Repository model object (which would break a lot of assumptions about repos always having parent projects etc)

The repo is created the first time it's accessed for a given Site. This happens synchronously for now, as it's a one time operation per Site (do we have other reasons than perf/scaling for the usual asynch repo creation?)

Didn't end up creating any db-persisted wiki reference in the Site model: The wikis repo name/path is simply derived from Site title, e.g. 'SITENAME-site-wiki' (I assumed that each Site coexisting on a Gitorious installation has a unique name, here.)

Adds separate views for Site-wide wikis. Started looking into reusing/DRYing views and controllers between Site wiki pages and regular Pages, but seemed too fiddly - there's lots of little differences in the paths created etc so it seemed shared partials would be very small/many/parameterized?

Backend git clone/push/pull of the site wiki repos also handled and tested.

DRYed up the wiki "wikization" code into its own module in lib.

Only setting up minimal prereceive hook for the site wikis, to avoid normal post push event processing which doesnt apply for site wiki.

Note: discovered that wiki page->edit->preview is broken in mainline (after jQuery upgrade or something?). Will look into it and create separate merge request once I figure out how our js asset pipeline works. :)