WIP: Add templates for issues at GitLab CE repository.

What does this MR do?

GitLab 8.11 now supports templates for issues and merge requests. It's quite strange that repositories for GitLab CE and EE still don't have those templates.

Contribution guidelines actually have two templates for issues. This request adds them to .gitlab/issue_templates/.

Comments were added to the CONTRIBUTION.md to remind about updating both templates and the guidelines document.

Are there points in the code the reviewer needs to double check?

The templates might be checked.

• Are they up-to-date? Do you often ask users to add something else?
• Are the names feature and bug right? Maybe call it feature request or proposal?
• Should any /commands be added?

Why was this MR needed?

• To improve the quality of issues, posted to GitLab CE and EE repositories;
• To make a living example of the new feature, introduced in 8.11.

Screenshots (if relevant)

You can test the feature at https://gitlab.com/nick.volynkin/gitlab-ce/issues/new

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?