normalize scss partials organization & naming

[clhenrick]

It's slightly confusing that all of mapd3's JS components are stored under ./src/charts/ but that the scss partials are split into ./src/styles/charts/ and ./src/styles/common/.

For example, the tooltip component is located in src/charts/tooltip.js but the equivalent scss partial is located in src/styles/common/_tooltip.scss.

It's also somewhat unclear why some scss partials are prefixed with an underscore and some are not.

This last point is just a nit-picky thing to me, and feel free to take or leave it, but IMO the point of entry for the scss partials should probably be named main.scss instead of mapd3.scss. The former is more of a standard / convention with scss.

We should clean this up to make it more consistent, it could be confusing to developers who want to contribute to this repository.