SMS/MMS: Switch to using IndexedDB

Right now, SMS Backup Reader 2 uses plain JS objects, arrays, and maps for storing SMS and MMS messages. This can cause issues with performance and scaling to larger backups. This issue is for switching to using IndexedDB.

Will likely follow https://developers.google.com/web/ilt/pwa/working-with-indexeddb and use the IDB library, since promises make more sense for this use case.