Session stores enrollment expiration, context processor makes available to templates

[thekaveman]

As part of #1877, we need to store the calculated expiration date in the user's session and make it available for use later on e.g. the Enrollment success page (#1919) and Enrollment error page (#1921).

Acceptance Criteria

• benefits.core.session defines a helper for getting the user's current expiration date as a datetime instance
• benefits.core.session.update takes a datetime parameter for updating the user's current expiration date
• A new context processor adds the expiration_date and renenrollment_date to the context for templates to use