Change invite according to the modified newcomer process

Newcomers will now be invited only when they select an issue. Then the maintainers will invite them and assign it to them. This means `invite me` has to be removed and the invite message for newcomers has to be changed. And "Hello world"s should not trigger invitations either.

The issue names no files, tests, or entry points. Locate the Python handlers for the `invite me` command, newcomer invite message, and `Hello world` matching, then review their existing tests if present. Done means removing the command, updating the message, and ensuring `Hello world` no longer triggers invitations.

Written by the indexing model from the issue text.