Implementation of APIv4: POST users/{user_id}/image (#5537)

* APIv4: POST users/{user_id}/image

* removed 'return' and rebased to master

i18n/en.json

@@ -3559,6 +3559,14 @@
     "id": "model.client.read_file.app_error",
     "translation": "We encountered an error while reading the file"
   },
+  {
+    "id": "model.client.set_profile_user.no_file.app_error",
+    "translation": "No file under 'image' in request"
+  },
+  {
+    "id": "model.client.set_profile_user.writer.app_error",
+    "translation": "Unable to write request"
+  },
   {
     "id": "model.command.is_valid.create_at.app_error",
     "translation": "Create at must be a valid time"