Move data source up to App so it can be set to "custom" when we set the API mode to local

[adamnovak]

Right now when you switch to the local API, you also have to switch the mode to custom tracks yourself.

We should do that automatically. But it is controlled inside HeaderForm right now, and we need to move it up into App if we want to control it from the component where we set the API.

None of the other presets work with the local API right now (except maybe built-in examples?), so we should maybe just lock it to custom when you are on the local API.

Also, maybe we could make local API actually a sibling option to custom, instead of its own setting, splitting custom into server-side and local custom modes? That might make it easier to link to.