Add competition creation allowlist via COMPETITION_CREATOR_GROUP

[cjh1]

This PR adds a configurable allowlist for competition creation using COMPETITION_CREATOR_GROUP.

Changes

• Added new setting in src/settings/base.py.
  • COMPETITION_CREATOR_GROUP (loaded from env, default empty).
• Added permission helpers and class in src/apps/api/permissions.py
• Updated competition create API permission in src/apps/api/views/competitions.py:
  • create now requires IsCompetitionCreator (instead of any authenticated user).
• Added enforcement in competition bundle completion flow in src/apps/api/views/datasets.py:
  • deny if configured creator group is missing.
  • deny if user is not allowed to create competitions.
• Exposed frontend capability flag in src/utils/context_processors.py.
• Updated UI buttons visibility.
• Documented env var in .env_sample.

Behavior

• If COMPETITION_CREATOR_GROUP is empty: authenticated users can create competitions.
• If set and group exists: only group members (and superusers) can create.
• If set but group does not exist: competition creation is disabled with explicit error messages.

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge