[PLT-7362] Option to add user to channel if mentioned user is not currently in the channel (#7619)

* Option to add user to channel if mentioned user is not currently in the channel

* instead of link from server, just add component on client side to add channel member

* change implementation using post.props

* do clean up and add test

* sanitize post.props['add_channel_member'] on post creation

* move sanitize to app.CreatePost and also apply to app.UpdatePost

i18n/en.json

@@ -1553,13 +1553,21 @@
     "id": "api.plugin.upload.no_file.app_error",
     "translation": "Missing file in multipart/form request"
   },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.link.public",
+    "translation": "add them to the channel"
+  },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.link.private",
+    "translation": "add them to this private channel"
+  },
   {
     "id": "api.post.check_for_out_of_channel_mentions.message.multiple",
-    "translation": "{{.Usernames}} and {{.LastUsername}} were mentioned, but they did not receive notifications because they do not belong to this channel."
+    "translation": "@{{.Usernames}} and @{{.LastUsername}} were mentioned but they are not in the channel. Would you like to {{.Phrase}}? They will have access to all message history."
   },
   {
     "id": "api.post.check_for_out_of_channel_mentions.message.one",
-    "translation": "{{.Username}} was mentioned, but they did not receive a notification because they do not belong to this channel."
+    "translation": "@{{.Username}} was mentioned but they are not in the channel. Would you like to {{.Phrase}}? They will have access to all message history."
   },
   {
     "id": "api.post.create_post.attach_files.error",