- Added space to li definiton
- Remove milestone from import_export.yml

- Added Importable to models/list.rb
- Did unless: :importable? on board validation
- Created changelog
- Modified haml to show issue boards are importable
- Added needed spec tests
- Modified project.json to include board information
- Added relevant models to all_models
- Added relevant models to import_export
- Added relevant models to safe_model_attributes

Prioritize and simplify project settings content.

Signed-off-by: Rémy Coutable <remy@rymai.me>

We dropped Webhooks from import/export in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24121,
but the online help still shows it's there.

The LFS files will be included in the `lfs-objects` directory in the archive.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>