[UX] Add a message when the registration form/link is not available

Problem/Motivation

Currently, if the registration form/link is not available for any reason (event full, registrations closed or disabled, not enough permissions, etc.) the field appears empty so the user cannot understand what to do.

Proposed resolution

If registrations are closed, output a message that explains why, using the errors generated by the registration_status() function.
If registrations are open but there is a permission problem, output an other message:
- For anonymous: Login or create an account to register.
- For authenticated people: You are not allowed to register.

I suggest to add a checkbox to the formatters settings to allow the site builders to disable this message if they don't want it.

Remaining tasks

Patch, Review, Commit

User interface changes

New checkbox in the formatters settings.
New message that replaces registration form or link under certain circumstances.

API changes

None.

Data model changes

A new formatter setting to store.