[refactor] Remove Topic from api syncer stores

[keyserj]

Since addressing #556 , the only reason for storing Topic in the api syncer stores is so that we know which Topic to make the changes to when creating the API request.

Storing the Topic is annoying because we have to ensure it's always correct, matching our current route. There are no plans to edit Topics from pages that aren't the Topic's page, so it doesn't seem worth doing this - we should be able to instead get the Topic from the current URL when creating the API request. If we want to edit Topics from a different page than their own page, we can figure that out later.