Native issue description template for epics

Problem

• How to better help users leverage lists in issue descriptions to function as epics.
• This can also serve as parent issues.

Proposed solution

• This is a continuation of the discussion here: https://gitlab.com/gitlab-org/gitlab-ce/issues/29444
• An issue description template that represents an "epic" that has a few sections of lists with obvious headings, for example: "child issues", or "dependent issues", "blocking issues", etc.
• The epic description template would be stored in the database, similar to the default one here: https://gitlab.com/help/user/project/description_templates#setting-a-default-template-for-issues-and-merge-requests
• This should not be managed in the repo itself, because we would need it to be usable out of the box, without any set up.
• The epic description template could then be changed by users in the project settings.
• This approach would become more obvious to users as this is completed https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7951.
• In the future, we would build additional features in the issue description view itself for lists, to delete list items, re-arrange them, etc., without
• Further off, we could on more native solutions : https://gitlab.com/gitlab-org/gitlab-ce/issues/4058.

Example of an epic issue template

### Problem
State the problem you are trying to solve

### Child Issues
* [ ] Child Issue Link e.g. #123
* [ ] Child Issue Link e.g. #456 
 
### Blocking Issues
* [ ] Blocking Issue Link e.g. #123
* [ ] Blocking Issue Link e.g. #456 

### Related Issues
* [ ] Related Issue Link e.g. #123
* [ ] Related Issue Link e.g. #456