Add testnet warning (#880)

* Add testnet warning

* Add watermarks to posts

* Call the test environment the Sandbox

src/view/com/util/PostSandboxWarning.tsx

@@ -0,0 +1,32 @@
+import React from 'react'
+import {StyleSheet, View} from 'react-native'
+import {Text} from './text/Text'
+import {useStores} from 'state/index'
+import {usePalette} from 'lib/hooks/usePalette'
+
+export function PostSandboxWarning() {
+  const store = useStores()
+  const pal = usePalette('default')
+  if (store.session.isSandbox) {
+    return (
+      <View style={styles.container}>
+        <Text type="title-2xl" style={[pal.text, styles.text]}>
+          SANDBOX
+        </Text>
+      </View>
+    )
+  }
+  return null
+}
+
+const styles = StyleSheet.create({
+  container: {
+    position: 'absolute',
+    top: 6,
+    right: 10,
+  },
+  text: {
+    fontWeight: 'bold',
+    opacity: 0.07,
+  },
+})