Fix slash commands detection in comments

What does this MR do?

Fixes how slash commands are identified in comment content (slash commands are always present in the beginning of line).

Why was this MR needed?

Due to a bug in how slash commands are identified, introduced with feature https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10760, any comment that has / somewhere in middle is stripped incorrectly, this MR fixes the way slash commands are detected so that comment string is not stripped incorrectly.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

EE port for this MR is available here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1876

What are the relevant issue numbers?

Closes #32016 (closed)