Fix slack command issues

What does this MR do and why?

Addresses some items from #384226 (closed)

Resolves the issue of missing [project name or alias] prefix for slash commands.

Resolves dispatch_failed when just the slash command is used.

Screenshots or screen recordings

Screen_Recording_2023-01-25_at_10.34.37_AM

How to set up and validate locally

1. Setup an environment by following steps mentioned in https://gitlab.com/gitlab-org/manage/integrations/team/-/blob/main/integrations/slack.md
2. Use the slash command with no command /dev-gitlab.
3. Use the slash command with an incorrect command /dev-gitlab sdfsdf.

MR acceptance checklist

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