REST API: Add partnerships parameter to petition update endpoint (#159)

source/includes/authenticated_api/_petitions.md.erb

@@ -410,6 +410,7 @@ external_site_locked | Boolean | Petition creator cannot change the external sit
 hide_petition_creator | Boolean | Do not display the petition creator's name or profile picture on the petition page
 hide_recent_signers | Boolean | Do not display the list of recent signers on the petition page
 hide_signature_form | Boolean | Turn off the form that allows people to sign the petition via the petition page
+partnerships | Array | List of partnerships this petition should be associated with, formatted like `[{"slug": "foo"}, {"slug": "bar"}]`. If the petition was associated with any partnerships that are not included in the list, it will be removed from those partnerships.
 petition_creator_name_override | String | Name to display for the petition creator on the petition page. Set to `null` to use the name from the petition creator's user account.
 redirect_to | String | If set, visitors to the petition page will be redirected to this URL
 show_progress_bar | Boolean | Whether to show the signature progress bar on the petition page