Implement new media send flow.

Update our media send flow to allow users to send multiple images/videos
at once. This change includes:

- New in-app media picker flow.
- Ability to caption images and videos.
- Image editing tools are made more prominent in the flow.
- Some fixes to the image editing tools.

res/drawable/mediapicker_item_border_light.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:shape="rectangle">
+
+    <stroke
+        android:width="1px"
+        android:color="@color/transparent_black_30" />
+
+</shape>