Added Consent functionality

[JeremySkinner]

This PR begins the implementation of Consent functionality by doing the following:

1. Adds a ConsentAgreement entity. This entity allows the definition of different consent agreements. They have a title, type (implicit/explicit), description and long description. They are revisionable:

2. Adds a new 'GDPR Consent' field type. This allows you to create new fields for requesting consent that can be attached to any form:

When defining this field, you select the agreement that you want the user to have to accept:

Some work will need to be done to align/style these fields at some point.

When completing the form, the agreement details are presented on the screen. The "description" shows as the label for the checkbox (if the agreement requires Explicit agreement).

Clicking the question mark shows the full text of the agreement inline:

[JeremySkinner]

cc @jamieaa64

[JeremySkinner]

@jamieaa64

Latest changes add support for identifying which user the consent applies to and adds appropriate permissions to give consent on behalf of another user. Notes are now also shown when editing consent on someone else's behalf:

Implicit consent is now also supported:

[JeremySkinner]

@jamieaa64 I've added the log functionality:

2 dashboard blocks available. The first lists the agreements that the user has accepted and the date they did so.

The second is a view that provides the full log. A log entry is written when an agreement is initially accepted by the user, or if the consent changes from yes/no or vice versa.