Seed interest groups and add interest event dev fixtures

[ch0rizo]

Description

Development seed data for interest events - nothing here touches production data.

• development_interest_groups.py seeds 20 fake interest groups (3 inactive) to fill the
  group grid, loaded only with load_fixtures --development. High pks (9070+) so they can
  never collide with real groups; initial_abakus_groups is untouched.
• Development fixtures: interest events, pools, registrations and leader memberships,
  referencing the four real interest groups from the initial tree
• load_fixtures date offsets keep the seeded events relative to now
• Adds the interest group info flatpage to initial_pages.yaml (real content - must be
  created manually in prod, which never runs load_fixtures)

Verified with a full migrate + load_fixtures --development run on a fresh database.
to now.

Testing

• The code quality is at a minimum required level of quality, readability, and performance.
• I have thoroughly tested my changes.

Please describe what and how the changes have been tested, and provide instructions to reproduce if necessary.

Resolves #4054

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