FIX: restore category when opening draft

This is a fix for the issue described in this [bug on meta-discourse](https://meta.discourse.org/t/new-topic-category-not-saved-as-part-of-draft/12277).

CategoryId exists in the composer after this line:
`composer = Discourse.Composer.loadDraft(opts.draftKey, opts.draftSequence, opts.draft);` because `loadDraft` makes a call to `#open`, loading the category. 

However, the second call to `composer.open(opts);` resets `composer`, this time without a category, because the opts hash doesn't directly contain a categoryId (it is nested inside of `opts.draft`).

Please provide any feedback! Another option I see would be pushing the logic from `.loadDraft` into `#open` and removing `.loadDraft` so we can make a single call. If that is a better option, let me know and I can give it a shot.