fix: keep owner characters when token is only temporarily invalid

[ErikKalkoken]

• Characters are no longer disabled when their token can't be refreshed for the moment, for example during an EVE SSO outage.
• Syncing recovers by itself once the outage is over, so admins don't have to re-enable characters by hand.
• Characters that no longer have a usable token are removed, and the user and admins are notified, as already happens when a character leaves the corporation.

Fixes #38

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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