Add event and related models

[Shivangi-ch]

Fixes #9104

Short description of what this resolves:

Added event and related models

Changes proposed in this pull request:

• created a new events app and added the Event, EventType, DiscountCode, and Group models inside it
• added EventSubTopic model inside the existing event_topics app.

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream dev-v3 branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[Shivangi-ch]

@norbusan @marcoag Could you please review the PR.