Update /links API endpoints to account for different types of links

[egiurleo]

I believe that this endpoint should be updated to take a type parameter. Depending on the type (resource_link, honorable_mention, or innovative_policy_idea), a different type of Link is created.

We may also want to update the PUT /links/_id endpoint to not allow changing the type of Link (not sure if that's even possible in the first place).

Otherwise, I believe that the links endpoints can be left alone. We may want to introduce type-specific APIs but I think that can be a separate ticket.