S5.6 add option for deleting connections

[anthony2261]

Issue #, if available: Closes #86

Description of changes:

• Add on delete: cascade to all foreign keys
• Remove ConversationMessage Association table, link Message table directly to Conv table via FK
• Add function on the backend to delete Connection and make accessible via endpoint
• Consume above endpoint on the frontend via a Delete button on the Connection Settings page

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[RamiAwar]

Niceee