Fixes slack livewire translations, UI

[snipe]

This PR just implements our existing translations for Slack and cleans up the UI a bit, and makes the spinner a little bigger and makes it actually spin. :)

[what-the-diff]

• Added translation for error message in SlackSettingsForm.php
• Updated the slack-settings-form blade file to use bootstrap classes and added fadeIn class on alerts
• Fixed a bug where testSlack() function was not returning anything when there is an exception thrown by GuzzleHttp\Client::post(). This caused the page to hang indefinitely after clicking Test Slack button if any of endpoint, channel or botname fields are empty (or invalid). Now it returns session flash messages as expected with appropriate translations applied from resources/lang/en/admin/settings/* files