Add a custom commit message when applying a suggested change

https://gitlab.com/gitlab-org/gitlab-ce/issues/18008 bring the ability to suggest a change from a comment. It would be helpful to the developer to customize the commit message when applying the suggested change.

Proposal

• When clicking on the "Apply suggestion" button, the user should be prompted for a custom commit message.
• The commit message is optional and the form will have a placeholder commit message as a default.
• Placeholder commit message: Apply suggestion on <filename>