feat: add InvitableMixin to GuildForum

[AstreaTSS]

You can invite people to a forum, after all.

Pull Request Type

• [x] Feature addition
• [ ] Bugfix
• [ ] Documentation update
• [ ] Code refactor
• [ ] Tests improvement
• [ ] CI/CD pipeline enhancement
• [ ] Other: [Replace with a description]

Description

Simply adds in InvitableMixin to GuildForum since you can indeed invite people to it.

Changes

See description and diff.

Related Issues

Fixes #1592.

Test Scenarios

await forum.create_invite()

Python Compatibility

• [ ] I've ensured my code works on Python 3.10.x
• [x] I've ensured my code works on Python 3.11.x

Checklist

• [x] I've run the pre-commit code linter over all edited files
• [x] I've tested my changes on supported Python versions
• [ ] I've added tests for my code, if applicable
• [ ] I've updated / added documentation, where applicable