Add delete site

[BraunRudolf]

Pull Request

Description

Add route where user can delete site

• check if user has access to site
• deletion is handled by pv_datamodel.write.user_and_site.delete_site this wasn't validated again

Fixes #141

How Has This Been Tested?

Unit-Test implemented

• [x] Yes

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[peterdudfield]

@all-contributors please add @BraunRudolf for code

[allcontributors[bot]]

@peterdudfield

I've put up a pull request to add @BraunRudolf! :tada: