Add sticker support.

No sticker packs are available for use yet, but we now have the
latent ability to send and receive.

test/unitTest/java/org/thoughtcrime/securesms/linkpreview/LinkPreviewUtilTest.java

@@ -2,6 +2,10 @@ package org.thoughtcrime.securesms.linkpreview;
 
 import org.junit.Test;
 
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import static junit.framework.TestCase.assertEquals;
 import static junit.framework.TestCase.assertFalse;
 import static junit.framework.TestCase.assertTrue;