Changed /authentication/reset_all_passwords to return 204 response code

refs https://github.com/TryGhost/Toolbox/issues/308

- this endpoint has no body to return but it was initially implemented
  as returning a 200 which we couldn't change until 5.0
- this changes it to a 204, removes the serializer and updates tests to
  take this into account

core/server/api/canary/authentication.js

@@ -221,6 +221,7 @@ module.exports = {
     },
 
     resetAllPasswords: {
+        statusCode: 204,
         permissions: true,
         async query(frame) {
             await userService.resetAllPasswords(frame.options);

core/server/api/canary/utils/serializers/output/authentication.js

@@ -47,10 +47,6 @@ module.exports = {
         };
     },
 
-    resetAllPasswords(data, apiConfig, frame) {
-        frame.response = data;
-    },
-
     acceptInvitation(data, apiConfig, frame) {
         debug('acceptInvitation');
 

test/regression/api/admin/__snapshots__/authentication.test.js.snap

@@ -449,14 +449,12 @@ Object {
 }
 `;
 
-exports[`Authentication API Reset all passwords reset all passwords returns 200 1: [headers] 1`] = `
+exports[`Authentication API Reset all passwords reset all passwords returns 204 1: [headers] 1`] = `
 Object {
   "access-control-allow-origin": "http://127.0.0.1:2369",
   "cache-control": "no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0",
-  "content-length": "2",
-  "content-type": "application/json; charset=utf-8",
   "etag": StringMatching /\\(\\?:W\\\\/\\)\\?"\\(\\?:\\[ !#-\\\\x7E\\\\x80-\\\\xFF\\]\\*\\|\\\\r\\\\n\\[\\\\t \\]\\|\\\\\\\\\\.\\)\\*"/,
-  "vary": "Origin, Accept-Encoding",
+  "vary": "Origin",
   "x-powered-by": "Express",
 }
 `;

test/regression/api/admin/authentication.test.js

@@ -478,12 +478,11 @@ describe('Authentication API', function () {
             mockManager.restore();
         });
 
-        it('reset all passwords returns 200', async function () {
+        it('reset all passwords returns 204', async function () {
             await agent.post('authentication/reset_all_passwords')
                 .header('Accept', 'application/json')
                 .body({})
-                // @TODO: change to a 204 in 5.0
-                .expectStatus(200)
+                .expectStatus(204)
                 .expectEmptyBody()
                 .matchHeaderSnapshot({
                     etag: anyEtag