Add component to display hints above action buttons

This component can be placed right above an ActionButtonFooter to display a "turquoise" colored, slightly smaller text, e.g. to give helpful hints what to do before saving. If put outside the ScrollView, it will permanently display and cover up part of the scrollable area until the area is scrolled.

Issue: 178

components/app-text.js

 import React from 'react'
-import { Text } from 'react-native'
+import { Text, View } from 'react-native'
 import styles from "../styles"
 
 export default function AppText(props) {
@@ -21,6 +21,18 @@ export function AppTextLight(props) {
   )
 }
 
+export function ActionHint(props) {
+  return (
+    <View
+      style={styles.actionHintWrappingView}>
+      <AppText
+        style={[styles.actionHint, props.style]}>
+        {props.children}
+      </AppText>
+    </View>
+  )
+}
+
 export function SymptomSectionHeader(props) {
   return (
     <AppText style={styles.symptomViewHeading}>

styles/index.js

@@ -17,6 +17,7 @@ const fontRegular = 'Prompt-Light'
 const fontLight = 'Prompt-Thin'
 
 const regularSize = 16
+const hintSize = 14
 
 const defaultBottomMargin = 5
 const defaultIndentation = 10
@@ -34,6 +35,14 @@ export default StyleSheet.create({
     fontFamily: fontLight,
     fontSize: regularSize
   },
+  actionHintWrappingView: {
+    margin: defaultIndentation
+  },
+  actionHint: {
+    color: secondaryColor,
+    fontFamily: fontRegular,
+    fontSize: hintSize
+  },
   paragraph: {
     marginBottom: defaultBottomMargin
   },