Feature: Send user invite

[madsrasmussen]

This PR adds support for inviting a user and resending an invite for an invited user.

How to test:

• Run the backoffice against the real server (no mock data)
• Enable the SMTP settings in appsettings.Development.json
• If you don't have a local setup for email testing you can install this: https://github.com/rnwood/smtp4dev/wiki/Installation

What to test:

• Test that you can invite a new user
• Test that you can resend an invite to a user

Known issues:

• You have to manually refresh the user list after an user is invited
• You can't complete the full invite flow. This PR only covers sending the invites.

[nielslyngsoe]

looks good