MM-10516: Added support for PostActions in ephemeral posts (#10258)

* Added support for PostActions in ephemeral posts

The general approach is that we take all the metadata that DoPostAction
needs to process client DoPostActionRequests, and store it in a
serialized, encrypted Cookie field, in the PostAction struct.

The client then must send it back, and it is then used to process
PostActions as a fallback top the metadata in the database.

This PR adds a new config setting, `ServiceSettings.ActionCookieSecret`.
In a cluster environment it must be the same for all instances.

- Added type PostActionCookie, and a Cookie string to PostAction.
- Added App.AddActionCookiesToPost.
- Use App.AddActionCookiesToPost in api4.createEphemeralPost,
  App.SendEphemeralPost, App.UpdateEphemeralPost.
- Added App.DoPostActionWithCookie to process incoming requests with
  cookies. For backward compatibility, it prefers the metadata in the
  database; falls back to cookie.
- Added plugin.API.UpdateEphemeralPost and plugin.API.DeleteEphemeralPost.
- Added App.encryptActionCookie/App.decryptActionCookie.

* Style

* Fixed an unfortunate typo, tested with matterpoll

* minor PR feedback

* Fixed uninitialized Context

* Fixed another test failure

* Fixed permission check

* Added api test for DoPostActionWithCookie

* Replaced config.ActionCookieSecret with Server.PostActionCookieSecret

Modeled after AsymetricSigningKey

* style

* Set DeleteAt in DeleteEphemeralPost

* PR feedback

* Removed deadwood comment

* Added EXPERIMENTAL comment to the 2 APIs in question

app/config.go

@@ -100,6 +100,64 @@ func (a *App) RemoveConfigListener(id string) {
 	a.Srv.RemoveConfigListener(id)
 }
 
+// ensurePostActionCookieSecret ensures that the key for encrypting PostActionCookie exists
+// and future calls to PostAcrionCookieSecret will always return a valid key, same on all
+// servers in the cluster
+func (a *App) ensurePostActionCookieSecret() error {
+	if a.Srv.postActionCookieSecret != nil {
+		return nil
+	}
+
+	var secret *model.SystemPostActionCookieSecret
+
+	result := <-a.Srv.Store.System().GetByName(model.SYSTEM_POST_ACTION_COOKIE_SECRET)
+	if result.Err == nil {
+		if err := json.Unmarshal([]byte(result.Data.(*model.System).Value), &secret); err != nil {
+			return err
+		}
+	}
+
+	// If we don't already have a key, try to generate one.
+	if secret == nil {
+		newSecret := &model.SystemPostActionCookieSecret{
+			Secret: make([]byte, 32),
+		}
+		_, err := rand.Reader.Read(newSecret.Secret)
+		if err != nil {
+			return err
+		}
+
+		system := &model.System{
+			Name: model.SYSTEM_POST_ACTION_COOKIE_SECRET,
+		}
+		v, err := json.Marshal(newSecret)
+		if err != nil {
+			return err
+		}
+		system.Value = string(v)
+		if result = <-a.Srv.Store.System().Save(system); result.Err == nil {
+			// If we were able to save the key, use it, otherwise ignore the error.
+			secret = newSecret
+		}
+	}
+
+	// If we weren't able to save a new key above, another server must have beat us to it. Get the
+	// key from the database, and if that fails, error out.
+	if secret == nil {
+		result := <-a.Srv.Store.System().GetByName(model.SYSTEM_POST_ACTION_COOKIE_SECRET)
+		if result.Err != nil {
+			return result.Err
+		}
+
+		if err := json.Unmarshal([]byte(result.Data.(*model.System).Value), &secret); err != nil {
+			return err
+		}
+	}
+
+	a.Srv.postActionCookieSecret = secret.Secret
+	return nil
+}
+
 // EnsureAsymmetricSigningKey ensures that an asymmetric signing key exists and future calls to
 // AsymmetricSigningKey will always return a valid signing key.
 func (a *App) ensureAsymmetricSigningKey() error {
@@ -209,6 +267,14 @@ func (a *App) AsymmetricSigningKey() *ecdsa.PrivateKey {
 	return a.Srv.AsymmetricSigningKey()
 }
 
+func (s *Server) PostActionCookieSecret() []byte {
+	return s.postActionCookieSecret
+}
+
+func (a *App) PostActionCookieSecret() []byte {
+	return a.Srv.PostActionCookieSecret()
+}
+
 func (a *App) regenerateClientConfig() {
 	clientConfig := config.GenerateClientConfig(a.Config(), a.DiagnosticId(), a.License())
 	limitedClientConfig := config.GenerateLimitedClientConfig(a.Config(), a.DiagnosticId(), a.License())