MM-45272: Fix getPostThread permissions (#20565)

Summary
Fix permissions for the the getPostThread API Method.
User can view thread if user is member of the channel
User can view threads in public channels (in the user's team) that they're not a member of, only if compliance export is disabled.

Ticket Link
https://mattermost.atlassian.net/browse/MM-45272

i18n/en.json

@@ -2237,6 +2237,10 @@
     "id": "api.post.check_for_out_of_channel_mentions.message.one",
     "translation": "@{{.Username}} did not get notified by this mention because they are not in the channel."
   },
+  {
+    "id": "api.post.compliance_enabled.join_channel_to_view_post",
+    "translation": "Due to compliance rules configured on this instance the channel must be joined before its posts can be read."
+  },
   {
     "id": "api.post.create_post.can_not_post_to_deleted.error",
     "translation": "Can not post to deleted channel."