update chesster db with slack user info before executing link command

[hopffgam]

due to rate limits we have to reduce the frequency in which chesster updates all channel and user information from slack. However, some of that data is required to execute the link command for a new player.

This functionality should be more robust: If a new player tries to link their account by executing the link command, and chesster does not find the required information in its database, it should query that data through the slack API for the specific user before continuing to process the link command.