Email integration

Enables the plugin to handle the attendee email. A new version of the database [10] is created and the setup script was modified to make the necessary updates in DB, whether the plugin is already installed or not, the DB will be ready for the email integration.

Currently, only the Attendees list and the New Attendee form use the email field.

There are a couple of styling refactor too, but will avoid this in the future to avoid unnecessary conflict with upstream development.