Add board_move slash command

What does this MR do?

Adds slash command board_move to move an Issue to a column on the board. Only supports projects with one board.

Are there points in the code the reviewer needs to double check?

• In the InterpretService where the slash command is implemented, there is a line to find all of the issue's labels that correspond to board lists. This query is almost identical to the one in app/services/boards/issues/move_service.rb in the #remove_label_ids method. Maybe this concept needs to be extracted somewhere? Might be overkill at this point, so I just left it for now.

• I added a lot of tests for various edge cases. I think that's good, but if it's going to cause trouble with the test suite performance, I could try to consolidate some of them.

Why was this MR needed?

To be able to move issues between boards from the issue page. See #28457 (closed)

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

What are the relevant issue numbers?

Closes #28457 (closed)