Change existing Slack slash commands endpoint route to match new events endpoint route

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Work on this issue
• Close this issue

This issue came out of this discussion #358676 (comment 934072419).

About

The API endpoint that processes Slack events #358676 (closed) is defined in API::Integrations::Slack::Events and the route is api/v4/integrations/slack/events.

The older API endpoint that processes Slack slash commands is related, but currently is defined in API::Integrations and the route is api/v4/slack/trigger.

Without any breaking changes, we would like to rename the class and route of the slash commands endpoint to match the events endpoint.

The benefits of this refactor is:

• Improved code consistency.
• Developers will understand the relationship more easily.
• Aids code discoverability.

Proposal

• Move the definition of the API endpoint to a new API::Integrations::Slack::SlashCommands class.
• Rename the route to api/v4/integration/slack/slash_commands (note: slash_commands and not trigger).
• Redirect the old route to the new route.
• Once deployed to .com: Update the GitLab (dev) Slack app and production Slack app to use the new route (don't publish the production Slack app)