Feature/643 delete endpoint

Describe the MR

Adds a button to delete endpoints in the endpoint card.

delete-endpoint

Why was this MR needed?

There was no way to delete endpoints.

Extra-care points?

There are many ways this can fail. Let's pray for the best right now.

Also, fixed a couple of bugs in the server code:

• Updating the list of checks on an alert with an empty list: (JSON empty array [] -> protobuf zero value for array -> Go zero value for array nil -> handler code did not discriminate between nil and [])
• In the rare case where the datapipe could not find a check in the database, we had a panic

Does this MR meet the acceptance criteria?

• Documentation created/updated
• CHANGELOG updated
• Unit Tests added/updated

What are the relevant issue numbers?

Closes #643 (closed)

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