dashboard for players who are not approved asks them to link

[glbert-does]

on the dashboard any unlinked player is asked to link their account:
https://github.com/Lichess4545/heltour/blob/master/heltour/tournament/templates/tournament/user_dashboard.html#L9-L19

unlinked players includes those who were not invited to slack, which is confusing for them. ideally, i think we'd add another condition to {% if not slack_linked %} – for example we could check for the existence of any season player object.
short term, maybe the text should just qualify with something like "to participate in the league, after being invited to our slack workspace, you need to link ..."