Displays navigable path name on file and folder pages.

This change displays the name of the source file at the top of the
page. The name can be copy/pasted directly into an editor or anything
else to get to the file. Fixes
https://bugzilla.mozilla.org/show_bug.cgi?id=758628
The individual components of the path can be clicked on for
convenient navigation.
Merge master in before I try to polish up clickable pathnames.
Polish pathname display for release.

* Use just '/' instead of os.sep. If we ran the server on Windows (which wouldn't work anyway), the path would come out as /treename/source\some\path.
* Tweak some naming, style, and spacing.
Add tests for linked_pathname(). Refactor it to be more testable.
Merge navigable pathnames. Fix #130. Fixes 758628.