1992 change host sponsor labels

[howsun7]

This accomplishes changes requested in #1992.

• Host should be renamed "Host Site" and should be required.
• Sponsor should be renamed "Organiser" and should be required.
• Membership can remain as is, and should be optional.

The code changes are done to make sure:

1. renamed values("Host Site" and "Organiser") are used in the event edit and creation forms.
2. the host and sponsor fields are required in forms; the membership field remains as optional.
3. the all-events and event detail views(templates) display the renamed values.

Note: the model field names are not changed; only the display(human-readable) names are.