Create event model and its children

[poju3185]

Fixes #9104

Short description of what this resolves:

Add the following models:

• discount_code_tickets
• discount_codes
• event_sub_types
• event_types
• events
• groups
• tickets

I add these models by following the schema here.

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development 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.

[poju3185]

This also fixes #9106

[Shivangi-ch]

I think instead of creating a separate app for each model, a better way would be to group functionally related models in the same app.
For example, models for event_sub_types, event_types, events, etc can be put in the same app instead of separate apps.
Having a separate app for each model would make the code structure messy and difficult to maintain.

One other way would be to make a models module and add all the models inside it.

@hongquan @marcoag what do you think which structure will work better for us?

[poju3185]

Hi @Shivangi-ch, thanks for your reply. I use different apps because I saw the old models are all in seperate apps. If everyone agree that we should put some of them in the same app, I am willing to do so.