Mandatory custom user field to provide consent

[nikosdion]

• [x] Frontend data protection options page (consent, export, delete)
• [x] Render a Joomla article in the protection page when clicking "Learn more", configurable in the backend.
• [x] System plugin for captive login. If consent is not given (or withdrawn) we only allow the frontend data protection options page.
• [x] Customizable allowed component / view / task combos for the captive login, prepopulated with LoginGuard views.
• [x] Audit log of last consent / withdrawal including user ID and IP address
• [x] Akeeba Subscriptions plugin to record consent when subscribing (so you don't have to go through a separate user registration process). The field ONLY validates when consent is given and then it's recorded in the audit trail.

[nikosdion]

Akeeba Subs does not need a new plugin. We just transcribe the user-provided consent from Akeeba Subscriptions to Data Compliance if no preference has yet to be recorded.