I've tried to come up with a patch that implements the "search
navigation" functionality in the classic tracker. What it does:

* The SearchAction creates a unique ID for each search and creates an
entry in the user's session that remembers the search URL for the
created unique search ID.

* The issue.index page stores the search result in the session, using
the search id as a key.

* The issue.item page, when coming from the issue.index page, adds
additional links to the page to:
 - go to the previous match
 - go to the next match
 - return to the search result

The search ID is passed on from the index page to the item page by
adding a variable to the GET request. This makes the URLs a bit ugly,
but is required so that multiple searches in multiple browser tabs don't
get in each other's way.

Please review & comment!