Read from Default.md for default issue and merge request description templates

• For default description templates, read from:
  • .gitlab/issue_templates/Default.md
  • .gitlab/merge_request_templates/Default.md
• When you create an issue or a merge request, the default template is already chosen in the dropdown, as shown below, and it is loaded into the view in the new issue / new merge request form.

• Disable the project settings UI so that the configuration text box is disabled, but you are still able to copy any existing text. You can no longer edit the text box contents though. It just displays whatever was already there before you upgrade GitLab to this version.
• Update messaging/UI as needed.
• The impact in this change is that users could be confused because their default templates seemingly disappeared, or changed (if they had a default.md already).
• This impact is offset by the messaging. And furthermore, since the previous template text is still available, an admin could quickly fix this by copying it.