SVNStorageRationale - from r526 to r527

This wiki uses SVN as its backend. This gives us a lot for 'free':

* Cheap versioning of pages and attachments with full history available.
* Authentication/authorization delegates to SVN which can be configured in myriad ways thanks to the magic of the ApacheWebServer integration.
* Recent changes is
equivalent to svn logroughly {{{svn log}}}, so is page and attachment history.
* The wiki data benefits from whatever backup/monitoring etc is in place for the SVN server.

All standard wiki features have been implemented with a SVN backend and a search index.