Set up mail server from settings page (5)

Instead of having your password in plain text in the application.yml file, it would be nice if we can use some runtime function to set the password at runtime when e-mails need to be sent. This allows an organizer to only provide the password as long as it is needed. Afterwards, the password is not saved anywhere.

This issue should probably created before #8 (closed).

• Create database table and migration for saving settings (1).
• Create repository for saving settings using generated JOOQ classes.
• Create page in the front-end for viewing and changing the settings (4).