Add FeatureFlag for mailbox

bramble-api/src/main/java/org/briarproject/bramble/api/FeatureFlags.java

@@ -11,6 +11,8 @@ public interface FeatureFlags {
 
 	boolean shouldEnableDisappearingMessages();
 
+	boolean shouldEnableMailbox();
+
 	boolean shouldEnablePrivateGroupsInCore();
 
 	boolean shouldEnableForumsInCore();

bramble-core/src/test/java/org/briarproject/bramble/test/TestFeatureFlagModule.java

@@ -25,6 +25,11 @@ public class TestFeatureFlagModule {
 				return true;
 			}
 
+			@Override
+			public boolean shouldEnableMailbox() {
+				return true;
+			}
+
 			@Override
 			public boolean shouldEnablePrivateGroupsInCore() {
 				return true;

briar-android/src/main/java/org/briarproject/briar/android/AppModule.java

@@ -31,6 +31,7 @@ import org.briarproject.bramble.plugin.tcp.AndroidLanTcpPluginFactory;
 import org.briarproject.bramble.plugin.tor.AndroidTorPluginFactory;
 import org.briarproject.bramble.util.AndroidUtils;
 import org.briarproject.bramble.util.StringUtils;
+import org.briarproject.briar.BuildConfig;
 import org.briarproject.briar.android.account.DozeHelperModule;
 import org.briarproject.briar.android.account.LockManagerImpl;
 import org.briarproject.briar.android.account.SetupModule;
@@ -340,6 +341,11 @@ public class AppModule {
 				return true;
 			}
 
+			@Override
+			public boolean shouldEnableMailbox() {
+				return BuildConfig.DEBUG;
+			}
+
 			@Override
 			public boolean shouldEnablePrivateGroupsInCore() {
 				return true;

briar-headless/src/main/java/org/briarproject/briar/headless/HeadlessModule.kt

@@ -107,6 +107,7 @@ internal class HeadlessModule(private val appDir: File) {
         override fun shouldEnableImageAttachments() = false
         override fun shouldEnableProfilePictures() = false
         override fun shouldEnableDisappearingMessages() = false
+        override fun shouldEnableMailbox() = false
         override fun shouldEnablePrivateGroupsInCore() = false
         override fun shouldEnableForumsInCore() = true
         override fun shouldEnableBlogsInCore() = true