Encrypt local sqlite store

[tusharm]

The local sqlite storage (that stores past visits) must be encrypted with a user-specific key - possibly a combination of user's UID and account creation time.

Notes:

• this will need a refactor since currently VisitsStore is init'd early on, even before user login information is available.
• will also be impacted by Historical visits stored locally should be scoped to the currently logged-in user #6 - different users must have different sqlite files.
• see https://pub.dev/packages/sqflite_sqlcipher