Update channel permissions for v4 endpoints (#5829)

* Fix join channel permission for v4 endpoint

* Allow regular users to get public channels they are not in

* Fix unit test

i18n/en.json

@@ -3111,6 +3111,14 @@
     "id": "authentication.permissions.create_group_channel.name",
     "translation": "Create Group Message"
   },
+  {
+    "id": "authentication.permissions.read_public_channel.description",
+    "translation": "Ability to read public channels"
+  },
+  {
+    "id": "authentication.permissions.read_public_channel.name",
+    "translation": "Read Public Channels"
+  },
   {
     "id": "authentication.permissions.create_team_roles.description",
     "translation": "Ability to create new teams"