RFC: Further reduction of controller boilerplate (Phase 2 of removing ja_resource)

We could do the following 3 items to further reduce boilerplate in controllers.

However, while boilerplate is reduced, all 3 of these actions also hide behavior which we may not want to hide.

### Remove `action_fallback CodeCorpsWeb.FallbackController` from all controllers and instead move it into the `CodeCorpsWeb` module into the `def controller` `quote` block

This makes sense at first glance, but it does also put all of our controllers into basically the same basket, regardless of them being actual API controllers, or just webhook handlers, or something else entirely.

Unfortunately, it is not currently possible to specify multiple action fallbacks for the same controller, which would be an argument for doing this, since it would allow simultaneously specifying global and group-specific fallbacks.

### Remove `plug CodeCorpsWeb.Plug.DataToAttributes` from all controllers and move it into `router.ex` into the `:api` pipeline.

We would have to separate our general api pipeline and the part of the pipeline that handles github and sprite webhooks, since we don't want this plug in that part, but it would make sense to do so anyway, since they are, in fact, different parts of the "interface".

### Remove `plug CodeCorpsWeb.Plug.IdsToIntegers` from all controllers and move it into `router.ex` into the `:api` pipeline.

Basically same argument as above.

Really, these two plugs should be considered one pipeline, since their intention is to prepare and make sense of the data sent by our client.

@JoshSmith

What do you think? Overall, this would not take much time, maybe an hour, but I'm not really sure if it would help or hurt us.