AllowEditPost and PostEditTimeLimit migration (#8208)

* AllowEditPost and PostEditTimeLimit migration

* Not set EDIT_POST permission to sysadmin_role if ALLOW_EDIT_POST is configured to NEVER

* Remove a bit of code duplication

config/default.json

@@ -45,7 +45,7 @@
         "RestrictCustomEmojiCreation": "all",
         "RestrictPostDelete": "all",
         "AllowEditPost": "always",
-        "PostEditTimeLimit": 300,
+        "PostEditTimeLimit": -1,
         "ExperimentalEnableAuthenticationTransfer": true,
         "TimeBetweenUserTypingUpdatesMilliseconds": 5000,
         "EnablePostSearch": true,