Follow-up from "Add slack_app_signing_secret to ApplicationSettings": Improve form labels for Slack application settings

The following discussion from !86623 (merged) should be addressed:

• @tomquirk started a discussion: (+1 comment)

  suggestion: could we mark this label for translation?

            = f.label :slack_app_signing_secret, s_('SlackIntegration|Signing Secret'), class: 'label-bold'

  I notice the other existing labels here aren't marked. Could add these in a follow-up but up to you 🙂