feature: Designate non-admin user who may post new entries.

This feature lets an authenticated user called blog-post use the POST /entry/add/ API endpoint to submit new entries. The user must be added to the database manually with an invitation code, and the invitation code must be used to authenticate the user first.

The main purpose behind this change is to allow the submit-entries-by-email script to work without having to divulge an admin user account name and password.