add a navbar button that opens the repo in the desktop's native file browser

This should work on linux (xdg-open) and OSX (open). If the program
is not in $PATH, it falls back to opening a browser window/tab with file:///

The only tricky bit is the javascript code, that handles clicking on the
link. This is to avoid unnecessary page refreshes. Until I added the
return false at the end, the <a>'s normal click event also fired, so two
file browsers opened. I have not checked portability extensively.

Assistant/WebApp.hs

@@ -63,7 +63,7 @@ selectNavBar :: Handler [NavBarItem]
 selectNavBar = ifM (inFirstRun) (return firstRunNavBar, return defaultNavBar)
 
 inFirstRun :: Handler Bool
-inFirstRun = isNothing . threadState <$> getYesod
+inFirstRun = isNothing . relDir <$> getYesod
 
 {- Used instead of defaultContent; highlights the current page if it's
  - on the navbar. -}