rename fill attribute

Summary:
We were using `fill={true}` as an attribute to make flexbox containers fill the entire available space.

However, `fill` is an HTML attribute (see: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/fill) This caused warnings printed to the console.

This diff renames the attribute to `grow` with is also more in line with the Flexbox terminology.

Reviewed By: priteshrnandgaonkar

Differential Revision: D10488389

fbshipit-source-id: ed8553c6203cdf6df94d26c731164ecec4c9fbd2

src/__tests__/__snapshots__/App.electron.js.snap

@@ -3,7 +3,6 @@
 exports[`Empty app state matches snapshot 1`] = `
 <div
   className="css-1si6n3e"
-  fill={true}
 >
   <div
     className="toolbar css-1u64wvw"
@@ -113,7 +112,6 @@ exports[`Empty app state matches snapshot 1`] = `
   </div>
   <div
     className="css-9qtipk"
-    fill={true}
   >
     <div
       className="css-1atbcdi"