Add forward button to MediaPreview

Closes #5833
Related to #4906
// FREEBIE

res/menu/media_preview.xml

@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <menu xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:app="http://schemas.android.com/apk/res-auto">
+    <item android:id="@+id/media_preview__forward"
+          android:title="@string/media_preview__forward_title"
+          android:icon="@drawable/ic_forward_white_24dp"
+          app:showAsAction="always"/>
     <item android:id="@+id/save"
           android:title="@string/media_preview__save_title"
           android:icon="@drawable/ic_save_white_24dp"

res/values/strings.xml

@@ -1237,6 +1237,7 @@
 
     <!-- media_preview -->
     <string name="media_preview__save_title">Save</string>
+    <string name="media_preview__forward_title">Forward</string>
 
     <!-- media_overview -->
     <string name="media_overview__save_all">Save all</string>

src/org/thoughtcrime/securesms/MediaPreviewActivity.java

@@ -166,6 +166,13 @@ public class MediaPreviewActivity extends PassphraseRequiredActionBarActivity im
     image.setImageDrawable(null);
   }
 
+  private void forward() {
+    Intent composeIntent = new Intent(this, ShareActivity.class);
+    composeIntent.putExtra(Intent.EXTRA_STREAM, mediaUri);
+    composeIntent.setType(mediaType);
+    startActivity(composeIntent);
+  }
+
   private void saveToDisk() {
     SaveAttachmentTask.showWarningDialog(this, new DialogInterface.OnClickListener() {
       @Override
@@ -192,8 +199,9 @@ public class MediaPreviewActivity extends PassphraseRequiredActionBarActivity im
     super.onOptionsItemSelected(item);
 
     switch (item.getItemId()) {
-    case R.id.save:         saveToDisk(); return true;
-    case android.R.id.home: finish();     return true;
+      case R.id.media_preview__forward: forward();    return true;
+      case R.id.save:                   saveToDisk(); return true;
+      case android.R.id.home:           finish();     return true;
     }
 
     return false;