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

plugin/plugintest/api.go

@@ -2124,6 +2124,35 @@ func (_m *API) SendEphemeralPost(userId string, post *model.Post) *model.Post {
 	return r0
 }
 
+// UpdateEphemeralPost provides a mock function with given fields: userId, post
+func (_m *API) UpdateEphemeralPost(userId string, post *model.Post) *model.Post {
+	ret := _m.Called(userId, post)
+
+	var r0 *model.Post
+	if rf, ok := ret.Get(0).(func(string, *model.Post) *model.Post); ok {
+		r0 = rf(userId, post)
+	} else {
+		if ret.Get(0) != nil {
+			r0 = ret.Get(0).(*model.Post)
+		}
+	}
+
+	return r0
+}
+
+// DeleteEphemeralPost provides a mock function with given fields: userId, post
+func (_m *API) DeleteEphemeralPost(userId string, post *model.Post) {
+	ret := _m.Called(userId, post)
+
+	if rf, ok := ret.Get(0).(func(string, *model.Post) *model.Post); ok {
+		_ = rf(userId, post)
+	} else {
+		if ret.Get(0) != nil {
+			_ = ret.Get(0).(*model.Post)
+		}
+	}
+}
+
 // SendMail provides a mock function with given fields: to, subject, htmlBody
 func (_m *API) SendMail(to string, subject string, htmlBody string) *model.AppError {
 	ret := _m.Called(to, subject, htmlBody)