Edit issue title and description UI

• Now that confidentiality and project moving are no longer in the main area of the issue page, we can clean up the design.
• In particular, some problems with current design: https://gitlab.com/gitlab-org/gitlab-ce/issues/33873.
• See mockup below as a proposed design.
• Remove the word "Issue" from the header. It's extraneous.
• Move the Delete button to the combo box in the header.
• Move the Edit, Save, Cancel buttons.
• The title field of the issue supports GFM. So it should be part of the preview.
• Re-arrange elements so that the "description" template purports to be a title+description template.
• If a template has not been configured or added (https://docs.gitlab.com/ce/user/project/description_templates.html, https://docs.gitlab.com/ee/user/project/description_templates.html), the dropdown should simply not exist.