Previosly, if the topic embed request had a missing username parameter,
and SiteSetting.embed_by_username was empty we would fail to create the
new topic and not show any errors.
Now we will fallback using the priority:
1. Username parameter
2. SiteSetting.embed_by_username
3. SiteSetting.site_contact_username
4. system user
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Feature to allow each imported post to be created using a different discourse
username. A possible use case of this is a multi-author blog where discourse
is being used to track comments. This feature allows authors to receive
updates when someone leaves a comment on one of their articles because each of
the imported posts can be created using the discourse username of the author.
