Title placeholder for issues, merge requests, and epics

• Currently, when you create an issue or merge request, the title textbox has no placeholder text.
• When you edit an issue (inline editing mode), there is placeholder text, that says Issue title.
• To make all the areas consistent and simplify, propose the same design for issues, merge requests, and epics:
  • All title textboxes should have placeholder text, and it should say Title.

gitlab-ce#39250 deals with changing the placeholder text for comments and/or issuable descriptions.