Export issues to CSV

EE follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6683

What does this MR do?

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

The CSV will be emailed from a background job after confirmation via modal dialog.

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)

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.
• Check performance with staging data for more production-like timings
• Frontend modal dialog includes number of issues
• Button and dialog match mockups on mobile
• Test excel, openoffice, apple numbers and google sheets for compatibility
• Date format should be compatible with excel
• Background job sends CSV as email
• Email copy and styling

Followup MRs

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

• Ability to download CSV immediately in the foreground when the number of issues is small
• 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

What are the relevant issue numbers?

• Closes #1480 (closed)
• Also related to gitlab-org/gitlab-ce#4196, gitlab-org/gitlab-ce#5747 and https://gitlab.com/gitlab-org/gitlab-ee/issues/471