SAK-31709 Correctly handle numeric Site IDs (#6435)
Where a site's ID was numeric, the favorites JS was sending the Site
ID through as an integer rather than a string, causing an exception on
the server side. Fixed this as follows:
* Have the FavoritesHandler code (server-side) coerce all incoming
Site IDs to Strings.
* Modify the jQuery code to use `.attr()` instead of `.data()` when
retrieving Site IDs off DOM elements. This avoids any attempt to
coerce the stored values into JS types, which we didn't want
anyway.
(cherry picked from commit c148567113f383d2fe2b6c8740b3984410fb1d01)
