Implement /_synapse/admin/v1/register (#1911)

* Implement /_synapse/admin/v1/register

This is implemented identically to Synapse, so scripts which work
with Synapse should work with Dendrite.

```
    Test 27 POST /_synapse/admin/v1/register with shared secret... OK
    Test 28 POST /_synapse/admin/v1/register admin with shared secret... OK
    Test 29 POST /_synapse/admin/v1/register with shared secret downcases capitals... OK
    Test 30 POST /_synapse/admin/v1/register with shared secret disallows symbols... OK
```

Sytest however has `implementation_specific => "synapse"` which stops these
tests from running.

* Add missing muxes to gobind

* Linting

cmd/dendrite-demo-libp2p/main.go

@@ -197,6 +197,7 @@ func main() {
 		base.Base.PublicFederationAPIMux,
 		base.Base.PublicKeyAPIMux,
 		base.Base.PublicMediaAPIMux,
+		base.Base.SynapseAdminMux,
 	)
 	if err := mscs.Enable(&base.Base, &monolith); err != nil {
 		logrus.WithError(err).Fatalf("Failed to enable MSCs")

cmd/dendrite-demo-pinecone/main.go

@@ -210,6 +210,7 @@ func main() {
 		base.PublicFederationAPIMux,
 		base.PublicKeyAPIMux,
 		base.PublicMediaAPIMux,
+		base.SynapseAdminMux,
 	)
 
 	wsUpgrader := websocket.Upgrader{

cmd/dendrite-demo-yggdrasil/main.go

@@ -154,6 +154,7 @@ func main() {
 		base.PublicFederationAPIMux,
 		base.PublicKeyAPIMux,
 		base.PublicMediaAPIMux,
+		base.SynapseAdminMux,
 	)
 	if err := mscs.Enable(base, &monolith); err != nil {
 		logrus.WithError(err).Fatalf("Failed to enable MSCs")

cmd/dendrite-monolith-server/main.go

@@ -149,6 +149,7 @@ func main() {
 		base.PublicFederationAPIMux,
 		base.PublicKeyAPIMux,
 		base.PublicMediaAPIMux,
+		base.SynapseAdminMux,
 	)
 
 	if len(base.Cfg.MSCs.MSCs) > 0 {

cmd/dendrite-polylith-multi/personalities/clientapi.go

@@ -33,7 +33,7 @@ func ClientAPI(base *setup.BaseDendrite, cfg *config.Dendrite) {
 	keyAPI := base.KeyServerHTTPClient()
 
 	clientapi.AddPublicRoutes(
-		base.PublicClientAPIMux, &base.Cfg.ClientAPI, accountDB, federation,
+		base.PublicClientAPIMux, base.SynapseAdminMux, &base.Cfg.ClientAPI, accountDB, federation,
 		rsAPI, eduInputAPI, asQuery, transactions.New(), fsAPI, userAPI, keyAPI, nil,
 		&cfg.MSCs,
 	)

cmd/dendritejs-pinecone/main.go

@@ -215,6 +215,7 @@ func main() {
 		base.PublicFederationAPIMux,
 		base.PublicKeyAPIMux,
 		base.PublicMediaAPIMux,
+		base.SynapseAdminMux,
 	)
 
 	httpRouter := mux.NewRouter().SkipClean(true).UseEncodedPath()

cmd/dendritejs/main.go

@@ -236,6 +236,7 @@ func main() {
 		base.PublicFederationAPIMux,
 		base.PublicKeyAPIMux,
 		base.PublicMediaAPIMux,
+		base.SynapseAdminMux,
 	)
 
 	httpRouter := mux.NewRouter().SkipClean(true).UseEncodedPath()