Add custom embed_by_username feature

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.
Merge branch 'add_custom_embed_by_username' of github.com:justinleveck/discourse into justinleveck-add_custom_embed_by_username

Conflicts:
	config/site_settings.yml
Merge branch 'justinleveck-add_custom_embed_by_username'