Add a slash command to move an issue between board columns

Design

Add a slash command which :

• First remove the label(s) corresponding to any column of the board that the issue is already in
• Attach the new label to the issue
• The syntax is: /board_move target_column board_specified.
• For example, if you want to move the issue to the column associated with label "in qa", you would type /board_move "in qa".
• If you want to move the issue to the "readyForDev" column, you would type /board_move readyForDev.
• Only applies to the CE case, as described: https://gitlab.com/gitlab-org/gitlab-ce/issues/28457#note_24091851.