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/api.go

@@ -287,6 +287,14 @@ type API interface {
 	// SendEphemeralPost creates an ephemeral post.
 	SendEphemeralPost(userId string, post *model.Post) *model.Post
 
+	// UpdateEphemeralPost updates an ephemeral message previously sent to the user.
+	// EXPERIMENTAL: This API is experimental and can be changed without advance notice.
+	UpdateEphemeralPost(userId string, post *model.Post) *model.Post
+
+	// DeleteEphemeralPost deletes an ephemeral message previously sent to the user.
+	// EXPERIMENTAL: This API is experimental and can be changed without advance notice.
+	DeleteEphemeralPost(userId string, post *model.Post)
+
 	// DeletePost deletes a post.
 	DeletePost(postId string) *model.AppError