* 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
