Add "destroy link" route.

[DFurnes]

What's this PR do?

This pull request adds Bert's final ability - to destroy! 😱 The DELETE /{link} endpoint removes the given short-link from Bertly's link table, in case we made a link by mistake.

How should this be reviewed?

There's some code, and there's a test! 💥

Any background context you want to provide?

🧨

Relevant tickets

References Pivotal #172799443.

Checklist

• [x] This PR has been added to the relevant Pivotal card.
• [x] Added appropriate feature/unit tests.

[weerd]

This pull request adds Bert's final ability - to destroy!

Awwww shit! Bert has reached his final form! Super Sayan Bert!