[Feature] Email notification for unknown sign-ins

[elesiuta]

• Gitea version (or commit ref): 1.14.0+dev-215
• Git version: N/A
• Operating system: N/A
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
• Log gist:

Description

Option to receive emails when a sign in is detected from a new location, like this https://docs.gitlab.com/ce/user/profile/unknown_sign_in_notification.html

This feature would probably depend on #5312 being implemented first