Feed interstitial tweaks (#4733)

* Swap interstitial positions

* Fix color

src/components/Button.tsx

···
       if (variant === 'solid' || variant === 'gradient') {
         if (!disabled) {
           baseStyles.push({
-            color: t.palette.white,
+            color: t.palette.contrast_100,
           })
         } else {
           baseStyles.push({

src/view/com/posts/Feed.tsx

···
         variant: 'default',
       },
       key: feedInterstitialType,
-      slot: 20,
+      slot: 40,
     },
     {
       type: followInterstitialType,
···
         variant: 'default',
       },
       key: followInterstitialType,
-      slot: 40,
+      slot: 20,
     },
   ],
 }