list now scrollable, reformats list handling

- urls are now auto-generated from android
  and no longer self-handled
- long-clicking a message starts decryption
  process, short clicking does nothing right
  now
- only first three messages are decrypted
  on app load, to improve performance
- fragment structure now inside a list view
  which makes the listview inside scrollable
  automatically

app/src/main/res/drawable/rounded_card.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<shape xmlns:android="http://schemas.android.com/apk/res/android">
+    <solid android:color="@color/cardview_shadow_start_color"/>
+    <stroke android:width="3dp" android:color="@color/cardview_dark_background" />
+    <corners android:radius="10dp"/>
+    <padding android:left="0dp" android:top="0dp" android:right="0dp" android:bottom="0dp" />
+</shape>