Implement email notifications for important user events

[charludo]

Motivation

It is currently the user's responsibility to check for events. Instead, they should be notified about things like:

• their offer has been accepted/rejected
• a new offer application has been submitted and is waiting for acceptance/rejection
• a new report has been submitted for an offer the user owns
• the board has requested changes after receiving reports
• ...

User Story

As any kind of user I expect to be notified when I am required to take actions.

Proposed Solution

Add email template(s) and a generic function that takes the subject/context and a user and sends a notification to their email address.

Alternatives

Additional Context

Screenshots