Add `start-backup` script

This helps running a Signal Desktop instance from a backup (copy of profile
folder) when testing functionality such as database migrations, import/export,
etc.

Usage:
```
BACKUP=development-backup-2018-04-04 PROFILE=development ./scripts/start-backup
```

WARNING: This deletes the original profile and overwrites it with backup.

scripts/start-backup

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+if [[ "$BACKUP" == "" ]]; then
+  echo "BACKUP environment variable is required"
+  exit 1
+fi
+
+if [[ "$PROFILE" == "" ]]; then
+  echo "PROFILE environment variable is required"
+  exit 1
+fi
+
+backupPath="$HOME/Library/Application Support/Signal-$BACKUP"
+profilePath="$HOME/Library/Application Support/Signal-$PROFILE"
+
+rm -rf "$profilePath" && \
+  cp -R "$backupPath" "$profilePath" && \
+  NODE_APP_INSTANCE="$PROFILE" yarn start