🐛 Fixed signing key identificaiton in JWKs

refs https://github.com/TryGhost/Team/issues/1640

- The signing key returned by `GET /ghost/.well-known/jwks.json` was missing an OPTIONAL `use: "sig"` attribute needed to identify it as a signing key in client libraries. E.g. pyton lib: "pyjwt" or node lib: "jwks-client"
- More about the "use" attribute at RFC7515 - https://www.rfc-editor.org/rfc/rfc7515#section-4.1.4

core/server/web/well-known.js

@@ -16,7 +16,20 @@ module.exports = function setupWellKnownApp() {
 
     wellKnownApp.get('/jwks.json', async (req, res) => {
         const jwks = await getSafePublicJWKS();
-        res.json(jwks);
+
+        // there's only one key in the store atm
+        // based on this setting all of the keys to have
+        // "use": "sig" property
+        const keys = jwks.keys
+            .map(key => ({
+                e: key.e,
+                kid: key.kid,
+                kty: key.kty,
+                n: key.n,
+                use: 'sig'
+            }));
+
+        res.json({keys});
     });
 
     return wellKnownApp;

test/e2e-server/__snapshots__/well-known.test.js.snap

@@ -8,6 +8,7 @@ Object {
       "kid": Any<String>,
       "kty": "RSA",
       "n": Any<String>,
+      "use": "sig",
     },
   ],
 }
@@ -15,7 +16,7 @@ Object {
 
 exports[`.well-known GET /jwks.json should return a JWKS 2: [headers] 1`] = `
 Object {
-  "content-length": "265",
+  "content-length": "277",
   "content-type": "application/json; charset=utf-8",
   "etag": StringMatching /\\(\\?:W\\\\/\\)\\?"\\(\\?:\\[ !#-\\\\x7E\\\\x80-\\\\xFF\\]\\*\\|\\\\r\\\\n\\[\\\\t \\]\\|\\\\\\\\\\.\\)\\*"/,
   "vary": "Accept-Encoding",