Change timestamp data type.

[gzfrozen]

Fix the "Year 2038 problem".

• Change the data type of last_used in twofactor table.
• Create migration files.
• Delete unnessary type convertion.
• Tested in SQlite, MySQL, Postgres.
  • Test method: add the two step authenticator to a account using the original code, then update the code to see if it can still login

I am not confident if it is the only "Year 2038 problem". If there are still other problems please let me know.

Fixes #4177

[dani-garcia]

Thanks! Had to resolve a small merge conflict but LGTM!