[MICROBA-1666] * Use bleach to sanitize user-provided content of bulk course emails before storing in the database. * Add new `BULK_COURSE_EMAIL_ALLOWED_HTML_TAGS` setting to enable configuration of allowed HTML tags in bulk course emails.
[MICROBA-1666] * Use bleach to sanitize user-provided content of bulk course emails before storing in the database. * Add new `BULK_COURSE_EMAIL_ALLOWED_HTML_TAGS` setting to enable configuration of allowed HTML tags in bulk course emails.