Add `embed_url` parameter to topic update method

Currently, the topic create method does accept an embed_url when creating a new topic. This was pointed out to me in this thread about embedding comments via javascript. This is not yet in the docs, but I submitted a PR to add it.

This request is to add the embed_url to the topic update method as well.

When I started this comment migration project, I was unaware that there was the option of creating a topic and providing an embed_url since the docs were wrong. As such, I have several hundred topics created without the embed_url and I would now like to set that property on them.

(As a side note, we are on an enterprise hosting plan for the forum in question, so I don’t have the ability to run any rake commands to finish this migration manually)