WIP: Export issues to CSV

What does this MR do?

Adds a Download CSV button to a project's issue index.

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

• Performance. What affect might this have on the overall system

Why was this MR needed?

Users wanted a way to easily share lists of issues outside of gitlab.

Examples included handing out paper copies in meetings and emailing a selection of issues.

Screenshots (if relevant)

UX mockups (from #5747 (closed))

Tasks

• Consider including related data. E.g. Author name instead of id.
• Obey currently applied filters
• Ensure any relational data is joined beforehand
• Basic profiling.
• Frontend link from issues page
• Frontend styling to match mobile mockup
• Test excel, openoffice, apple numbers and google sheets for compatibility
• Date format should be compatible with excel

Followup MRs

I'll create issues/MRs for the following improvements:

• CSV generated in the background for large numbers of issues. Should email link on completion and notify user this is happening.
• Ability to export all issues for a group as in https://gitlab.com/gitlab-org/gitlab-ee/issues/471

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 merge request performance guides
• 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?

• Closes #5747 (closed)
• Also related to #4196 (closed) and https://gitlab.com/gitlab-org/gitlab-ee/issues/471