Add bak defaultapp setting
Support for multiple default apps

If a default app isn't visible for the user, try the next one.
Else fallback to the "files" app.
Use getAbsoluteUrl for redirection URL

Also separate the function into getDefaultPageUrl() and
redirectToDefaultPage() to make it testable.
Added unit test for default app URL
Merge pull request #9334 from owncloud/defaultappfix

Default app fix