add api to convert a channel from public to private and restrict that to system_admin (#8655)

i18n/en.json

@@ -215,6 +215,14 @@
     "id": "api.channel.change_channel_privacy.public_to_private",
     "translation": "This channel has been converted to a Private Channel."
   },
+  {
+    "id": "api.channel.convert_channel_to_private.default_channel_error",
+    "translation": "This default channel cannot be converted into a private channel."
+  },
+  {
+    "id": "api.channel.convert_channel_to_private.private_channel_error",
+    "translation": "The channel requested to convert is already a private channel."
+  },
   {
     "id": "api.channel.create_channel.direct_channel.app_error",
     "translation": "Must use createDirectChannel API service for direct message channel creation"
@@ -331,18 +339,6 @@
     "id": "api.channel.leave.left",
     "translation": "%v left the channel."
   },
-  {
-    "id": "api.channel.post_convert_channel_to_private.create_post.error",
-    "translation": "Failed to post channel conversion to private message"
-  },
-  {
-    "id": "api.channel.post_convert_channel_to_private.retrieve_user.error",
-    "translation": "Failed to retrieve user while converting the channel from public to private"
-  },
-  {
-    "id": "api.channel.post_convert_channel_to_private.updated_from",
-    "translation": "%s converted the channel from public to private"
-  },
   {
     "id": "api.channel.post_update_channel_displayname_message_and_forget.create_post.error",
     "translation": "Failed to post displayname update message"