APIv4 POST /reactions (#6092)

* APIv4 POST /reactions

* update corresponding V3 endpoint

i18n/en.json

@@ -1703,10 +1703,18 @@
     "id": "api.reaction.list_reactions.mismatched_channel_id.app_error",
     "translation": "Failed to get reactions because channel ID does not match post ID in the URL"
   },
+  {
+    "id": "api.reaction.save_reaction.invalid.app_error",
+    "translation": "Reaction is not valid."
+  },
   {
     "id": "api.reaction.save_reaction.mismatched_channel_id.app_error",
     "translation": "Failed to save reaction because channel ID does not match post ID in the URL"
   },
+  {
+    "id": "api.reaction.save_reaction.user_id.app_error",
+    "translation": "You cannot save reaction for the other user."
+  },
   {
     "id": "api.reaction.send_reaction_event.post.app_error",
     "translation": "Failed to get post when sending websocket event for reaction"