Calling sharing_add_footer after (!) wp_print_footer_scripts in wp_footer to ensure jQuery dependancy

Currently `sharing_add_footer` is being attached to `wp_footer` with priority 10 which is **before** WordPress has printed the footer scripts as `wp_print_footer_scripts` is attached to `wp_footer` with priority 20.

`sharing_add_footer` echoes inline Javascript during [`display_footer()`](https://github.com/Automattic/jetpack/blob/8ec83606e531c70d963291b591c8aff682775e6e/modules/sharedaddy/sharing-service.php#L548) ([here is an example](https://github.com/Automattic/jetpack/blob/6c49efab93f08f68780575f43936b546db477d54/modules/sharedaddy/sharing-sources.php#L499)) which depends on jQuery being already available. This fails in cases when jQuery is enqueued in the footer.


The solution is to move [`sharing_add_footer`](https://github.com/Automattic/jetpack/blob/4ae9a0e4fe15877340da66edabb3235548e328c2/modules/sharedaddy/sharing-service.php#L618) to a later priority:

```
add_action( 'wp_footer', 'sharing_add_footer', 25 );
```

Start in modules/sharedaddy/sharing-service.php at display_footer() and sharing_add_footer(), then review their wp_footer priorities alongside wp_print_footer_scripts. Reproduce a footer-enqueued jQuery case and verify that the inline JavaScript runs after the dependency is available; done means sharing_add_footer uses the later priority described in the issue.

Written by the indexing model from the issue text.