Disallow users from accepting invitation twice

Send email notifications when invitations are accepted/confirmed

Update Vault to 2.8.0

Remove redundant information from README after moving it to the wiki

Add build script to get some Git info to later show in admin panel, and add some metadata to Cargo.toml

Remove config option for admin email, embdedded admin page, managed IO::Error, and added security and cache headers globally

In the future, maybe we can do the same with the `find_by_id` methods that return an Option.

Fixed delete user when 2FA is enabled, implemented delete user for admin panel, and the front-end part for invite user. Secured admin panel behind a configurable token.

Initial version of admin panel, list users and reload user list works. No serious auth method yet, password is 'token123'