Update ml id column sites

[Sukh-P]

Pull Request

Description

This change drops the uniqueness requirement for the ml_id column in the sites table. The create site function has been modified slightly to include ml_id as an input so this can be easily tested.

Checklist:

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

[Sukh-P]

@peterdudfield I have had a go at the change we talked about regarding the ml_id column in the site table, first time touching this repo so feel free to point out bits that may need changing or anything else

[peterdudfield]

Great start, one small point ive added.

Would you be able to run a alembic migration? Ill happily do this together with you, if you like