Bed-2143 chore: add warning detection for non-unique emails

[mistahj67]

Description

• Add warning logs for non-unique emails
• Fixed missing license files
• Added duplicate enforcement to create and update user endpoints
• Added handling to UI for email conflict
• Added warning display on Users tab of admin panel if duplicate emails are detected
• Updated db method to store lowercase emails to prevent case-sensitive edge cases

Motivation and Context

This PR addresses: BED-2143

Why is this change required? What problem does it solve?

How Has This Been Tested?

Locally

Types of changes

• Chore (a change that does not modify the application functionality)

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https://github.com/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

[wes-mil]

I believe this PR requires a migration to convert all existing email addresses to lowercase, as the check performed in IsNewEmail relies on a lowercase assumption