Allow event hosts to provide an external URL.

[Former user]

Problem: Creating an event has limited fields.

Situation: Gym owners may have a website that they want users to visit in order to pay for the competition.

Solution: Have a new field on event creation page for additional URL, allowing hosts to provide a sign-up link.

[Thomas McNaught]

Allow hosts to provide an external URL to display to ninjas after expressing interest in an event (closes #5)

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