Fixed potential slack webhook setting inconsistencies

[marcusmoore]

Previously, if a user set their webhook to Slack, then later hit the Clear & Save button, then later came back and re-entered their Slack settings, webhook_selected would not be saved as slack.

This means that checks for which webhook is selected in CheckoutableListener would fail since Slack wasn't "selected".

This PR addresses this by

1. Properly resetting an empty webhook_selected to slack within the SlackSettingsForm livewire component so that future updates through that form will work as expected
2. Adds a migration that resets webhook_selected to slack if the webhook settings are empty (caused by the user clicking Clear & Save or if the user had re-added Slack settings (webhook contains slack.com) but webhook_selected was not properly set to slack.

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fixes #16965