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

model/system.go

@@ -10,13 +10,14 @@ import (
 )
 
 const (
-	SYSTEM_DIAGNOSTIC_ID          = "DiagnosticId"
-	SYSTEM_RAN_UNIT_TESTS         = "RanUnitTests"
-	SYSTEM_LAST_SECURITY_TIME     = "LastSecurityTime"
-	SYSTEM_ACTIVE_LICENSE_ID      = "ActiveLicenseId"
-	SYSTEM_LAST_COMPLIANCE_TIME   = "LastComplianceTime"
-	SYSTEM_ASYMMETRIC_SIGNING_KEY = "AsymmetricSigningKey"
-	SYSTEM_INSTALLATION_DATE_KEY  = "InstallationDate"
+	SYSTEM_DIAGNOSTIC_ID             = "DiagnosticId"
+	SYSTEM_RAN_UNIT_TESTS            = "RanUnitTests"
+	SYSTEM_LAST_SECURITY_TIME        = "LastSecurityTime"
+	SYSTEM_ACTIVE_LICENSE_ID         = "ActiveLicenseId"
+	SYSTEM_LAST_COMPLIANCE_TIME      = "LastComplianceTime"
+	SYSTEM_ASYMMETRIC_SIGNING_KEY    = "AsymmetricSigningKey"
+	SYSTEM_POST_ACTION_COOKIE_SECRET = "PostActionCookieSecret"
+	SYSTEM_INSTALLATION_DATE_KEY     = "InstallationDate"
 )
 
 type System struct {
@@ -35,6 +36,10 @@ func SystemFromJson(data io.Reader) *System {
 	return o
 }
 
+type SystemPostActionCookieSecret struct {
+	Secret []byte `json:"key,omitempty"`
+}
+
 type SystemAsymmetricSigningKey struct {
 	ECDSAKey *SystemECDSAKey `json:"ecdsa_key,omitempty"`
 }