Option to record marketing consent per user

[ianroberts]

Related to #266

For most uses of Teamware I expect that the collection of user email addresses at signup would only be for technical reasons (e.g. to be able to send registration and password reset emails, or to send notifications directly related to the Teamware service such as planned outages and upgrades) and so would be covered by "user's legitimate interest" in GDPR terms. However some third parties deploying Teamware may want to use the captured emails for other purposes such as marketing, for which they need to gain the user's consent.

A full implementation of this would be quite complex but a starting point could be to add

• a configuration option (false by default) to add a checkbox to the signup form asking for opt-in to marketing emails
• a couple of fields on the user domain object to record whether and possibly when they ticked that box

This is nowhere near enough for a full consent management system but it would allow Teamware admins to capture the initial consent and import new users into a more fully-featured CRM tool.