Move GitLab for Slack-related routes and controller code to FOSS
This change is part of #355896 (closed) to move all GitLab for Slack app code to Core.
This MR focuses on moving:
- Routes
- Controllers
- Related views and helpers
A changelog entry, and any documentation changes, will be added in the final MR.
There should be no functional changes.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #355896 (closed)