Add more checkbox styles (#7058)

Add a "kind" param for StyledCheckbox, allowing designers to choose different styles of checkbox as needed.  In addition to the preexisting default kind (now called Solid), there's an Outline kind with a green checkmark and a transparent fill.  This is used in the device trust view, since the default checkbox style looks too much like the green "verified" shield and it's awkward to have those next to each other.

test/components/views/settings/__snapshots__/FontScalingPanel-test.tsx.snap

@@ -243,7 +243,7 @@ exports[`FontScalingPanel renders the font scaling UI 1`] = `
           onChange={[Function]}
         >
           <span
-            className="mx_Checkbox "
+            className="mx_Checkbox mx_Checkbox_hasKind mx_Checkbox_kind_solid"
           >
             <input
               checked={false}
@@ -258,8 +258,8 @@ exports[`FontScalingPanel renders the font scaling UI 1`] = `
               <div
                 className="mx_Checkbox_background"
               >
-                <img
-                  src="image-file-stub"
+                <div
+                  className="mx_Checkbox_checkmark"
                 />
               </div>
               <div>