Slack setup doesn't work if channels cache was recently cleared

[noahmoss]

Describe the bug
A slack channels cache was added in #20570 to improve performance. The cache is only refreshed if it was not refreshed within the last 10 minutes. It is also cleared whenever a Slack token is deleted. This means that if you've deleted a Slack app within the last 10 minutes, you can't add a new one, because the cache will be empty and the check that the files channel exists will fail.

To Reproduce
Steps to reproduce the behavior:

1. Create a new Slack integration in Metabase using the instructions in the admin panel
2. Delete the Slack integration
3. Add the Slack integration again
4. See a "channel not found" error on save, even though the files channel exists

Information about your Metabase Installation:
Current master

Severity
p1/2? only workaround is to wait 10 minutes and try again

[dpsutton]

@noahmoss Do we not populate the channel cache when saving a new token? The empty cache remains empty until its scheduled refresh?

[noahmoss]

@dpsutton The issue is that there's a function that checks whether the cache was updated within the last 10 minutes. We try to update it when a new token is saved, but if that check returns false, we skip it.