RESTify and resourceify the api/venues routes.

The PR brings the api/venues routes more in line with The Rails Way<sup>TM</sup>.

## Notable changes:

* Rename the controller and test to be plural instead of singular.  Api::VenuesController instead of Api::VenueController
* Use the standard `index` action instead of `list` (renamed the method in the controller, and moved the view template)
* Use the `resources` helper in `config/routes.rb`

## Potentially breaking change

* To show the venue with an `id` of 1 the URL will now be `api/venues/1` instead of `api/venues/show/1`.

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If this PR is accepted I'll create similar PRs for all of the other resources in the project that can be RESTified and resourceified.