Refactors the period selector component now that the design / system is
pretty much finished.
Main points are: change from using CSS selectors to using styled
components; use props instead of classes. This is in keeping with the
general Unleash approach.
There's two very slight visual changes here:
1. There is 4px of added space below the "range" "header" text.
2. The months in the grid are a little closer together and not as wide.
This is because we remove the explicit column gap due to the grid having
a set width. Previously the width was automatic, but because we want
this to line up with the button, we need to set the width explicitly on
both items. As such, with the padding, the grid was a little too wide,
so there was too little padding on the right. This rectifies that.
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