Song page behaviours

Some behaviours in the Songs page should be changed:

- After updating a song from the form and arriving at the song page, the back button in the browser should take you back to the search page. Currently, it takes you back to the form page. (This can be done by changing the `router.push` to a `router.back`.)
- After updating a song from the form and arriving at the song page, the song text (and author and melody) should be updated. Currently, it is not.