Add "lock conversation" for issues

[kolaente]

It would be cool if Gitea would support the "lock conversation" feature like github does.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[adelowo]

Bummer

[techknowlogick]

@adelowo the stale bot is a nudge for people involved in the tickets, so it doesn't mean this ticket is going away.