Export project functionality

This is a MR for the export functionality of https://gitlab.com/gitlab-org/gitlab-ce/issues/3050, which adds the ability to export single projects.

- [x] members
- DB data
  - [x] issues
  - [x] issue comments
  - [x] merge requests
  - [x] merge request diff
  - [x] merge request comments
  - [x] labels
  - [x] milestones
  - [x] snippets
  - [x] releases
  - [x] events
  - [x] commit statuses
  - [x] CI builds
- File system data
  - [x] Git repository
  - [x] wiki
  - [x] uploads
  - [ ] ~~CI build traces~~
  - [ ] ~~CI build artifacts~~
  - [ ] ~~LFS objects~~
- DB configuration
  - [x] services
  - [x] web hooks
  - [x] protected branches
  - [x] deploy keys
  - [x] CI variables
  - [x] CI triggers

See merge request !3114