Add author_name and provider_name to Embed

Some embed providers set author_name and provider_name which are sometimes useful to know (especially when they don't send the title).

This pull request adds these two fields to the Embed model to save these values in.

It also fixes a few bugs:
 - Fixed a crash when title field isn't set
 - Use templates for embed rendering
 - Use escapejs filter to make sure that embed code sent as a javascript string to the editor is valid
 - No longer shows broken image in the editor if no thumbnail_url was set

Thanks to @artursmet for spotting these issues.