Calculating Low, Medium and High gas price suggestions

[danjm]

On the send screen, cancel model and speed up / gas customization modal/screen, we will have a component that suggests three possible prices to the user.

We need to be able to calculate these either at the time at which they need to be rendered, or as blocks update.

The result of this task should be either gas price suggestion data within our redux state that is updated from the background on block changes, or a function that can be called to return the suggestions.

[danjm]

First cut at this can be found on https://github.com/MetaMask/metamask-extension/tree/mmui-i1-data-for-basic-estimates

I'll put a PR up once we get some blocking PRs merged