Add credential exposure tables and adjust foreign keys

[DJensen94]

Added data models for the postgres database for the different hibp and cybersix exposure sources. Also updated direction of the relationships in dns tables to point the correct direction
fixes #38

💭 Motivation and context

• This change is required to get the initial database configuration set up to model the data we are pulling from our various APIs that we will then use when generating our reports.
• This solves our problem of not having a model/ or location to store the data we need for our report

🧪 Testing

passes pre-commits and pytest

✅ Checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

[coveralls]

Pull Request Test Coverage Report for Build 1194454233

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 69.583%

---

Totals
Change from base Build 1174855813:	0.0%
Covered Lines:	157
Relevant Lines:	224

---

💛 - Coveralls

[schmelz21]

Looks good to me.

[schmelz21]

@DJensen94 , @aloftus23 make sure to add in assignee, labels, links and projects when creating PR.. makes it traceable the more we build out here.

[cduhn17]

This schema looks good.