Forbid users from creating DMs and GMs with remote users (#28855)

* Forbid users from creating DMs and GMs with remote users

This change prevents the application from creating new DMs and GMs
when remote users are involved, and prevents as well new posts from
being created into preexisting DMs and GMs with remote users created
in previous versions.

* Adds i18n

* Skip preexisting DM tests and create a new one for the new behavior

server/i18n/en.json

@@ -299,6 +299,10 @@
     "id": "api.channel.create_channel.direct_channel.app_error",
     "translation": "Must use createDirectChannel API service for direct message channel creation."
   },
+  {
+    "id": "api.channel.create_channel.direct_channel.remote_restricted.app_error",
+    "translation": "Cannot create a direct channel with a remote user"
+  },
   {
     "id": "api.channel.create_channel.direct_channel.team_restricted_error",
     "translation": "A direct channel cannot be created between these users because they do not share a team in common."
@@ -335,6 +339,10 @@
     "id": "api.channel.create_group.bad_user.app_error",
     "translation": "One of the provided users does not exist."
   },
+  {
+    "id": "api.channel.create_group.remote_restricted.app_error",
+    "translation": "Cannot create a group channel with remote users"
+  },
   {
     "id": "api.channel.delete_channel.archived",
     "translation": "%v archived the channel."
@@ -6154,6 +6162,10 @@
     "id": "app.post.cloud.get.app_error",
     "translation": "Can not fetch the post as it is past the cloud's plan limit."
   },
+  {
+    "id": "app.post.create_post.shared_dm_or_gm.app_error",
+    "translation": "Cannot create the post in a DM or GM with remote users"
+  },
   {
     "id": "app.post.delete.app_error",
     "translation": "Unable to delete the post."