Add model and form to create and edit exhibitors

[Shivangi-ch]

Fixes #9111

Short description of what this resolves:

Added model and form to create and edit exhibitors

Changes proposed in this pull request:

• Added model for Exhibitors inside events app.
• Added form to create and edit exhibitors in forms.py file

PS - This PR builds on top of work done in PR #9125. Please merge it before merging this PR.

Checklist

• [x] I have read the Contribution & Best practices Guide and my PR follows them.
• [x] My branch is up-to-date with the Upstream development branch.
• [x] The unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)
• [x] All the functions created/modified in this PR contain relevant docstrings.

[Shivangi-ch]

@hongquan Could you please review this PR

[hongquan]

Please add only one new migration file per app. Your new "events" app is adding 3 new migrations. It is redundant. Also please format your new code with Ruff.