Stats: Break up monolithic _stats.scss file into appropriate component files

[alternatekev]

As part of the stats maintenance effort underway, we need to take the assets/stylesheets/sections/_stats.scss file and break it down:

• It needs to be moved into the client/my-sites/stats/ directory
• The rules inside of it need to be shuffled into their various component-specific directories, which now exist.

This PR touches the following components:

• StatsChartTabs
• StatsComments
• StatsOverviewPlaceholder

In order to test:

• Verify that the tabs show up correctly on the Day/Week/Month/Year charts
• Verify that the comments list displays and functions correctly on the Insights page
• Verify that when localStorage is cleared, you get visual placeholders on the modules while they're loading from the API

[alternatekev]

It looks like #2384 is going to be covering the StatsChartTabs component SCSS so skip that for now.

[timmyc]

Did a quick pass of all the components loaded and in empty localstore state. I didn't see anything missing and since this is a copy and paste I wouldn't expect it ;)

So if you are also good with it Kevin lets get this merged in to prevent conflicts on other branches. End of an era :)