Add "Copy to clipboard" button in commit email selection box

The following discussion from gitlab-ce!22560 should be addressed:

• @jkarthik started a discussion: (+3 comments)

  I think we should probably make it a bit clearer in the current UI that the user needs to set this new email as their own in their local environment

  @tiagonbotelho I don't understand what you mean here. Can you please clarify?