Add the ability to POST a new link to the talks/42/links endpoint, with fields "link_type", "title" and "url". This should be saved in the talk_links table. Each resource should include a link to its own unique uri. Only speakers, event admins and site admins can do this.

Implement the individual endpoint so that it is possible to GET an individual talk_link resource. The URL should be /talk_links/[talk_link_id].

Allow editing of talk links by PUTting to the individual talk_link endpoint. The data should be the same as what is returned by the GET endpoint above but the title, type and URL fields can be changed.

Feel free to provide this as separate PRs