Modify GroupMembership view functions to support end_date field

[cc-a]

The main views that require updating are send_group_invite and accept_group_invite form.

For send_group_invite:

• Update GroupMembershipForm to include a DurationField specifying the length of the new users membership - use an arbitrary fixed length for now.

For accept_group_invite:

• Use the end date value decoded from the token when creating the GroupMembership instance.

Slightly tweaks to the above may be required depending on progress in #76 which will add at least one more view function into the user sign up process.