Redirect /import to project page if no importing at all and repo exists


Fixes gitlab-org/gitlab-ce#13367.

See merge request !2857

Handle nil commits in Gitlab::PushDataBuilder.build

Closes #13469

See merge request !2825

Fixes #13367.

This ensures the caches for Repository#empty? and
Repository#has_visible_content? are flushed after a repository has been
imported or forked.

Fixes gitlab-org/gitlab-ce#13505

Add new data to project in push, issue, merge-request and note webhooks data

_Originally opened at !2738 by @bugagazavr._

- - -

**What does this MR do?**

Adds more data to hooks, makes repository data in all hook more dry ( use one hook_attrs method for repository data in all hooks )

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

1. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/app/models/project.rb#L738
2. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/app/models/concerns/issuable.rb#L132
3. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/lib/gitlab/push_data_builder.rb#L52
4. https://gitlab.com/bugagazavr/gitlab-ce/blob/extend-hooks/lib/gitlab/note_data_builder.rb#L56

**Why was this MR needed?**

More information about user for push events, and more data about repositiry ( project )

**What are the relevant issue numbers?**

No

**Screenshots (if relevant)**

No

See merge request !2788

Fix cross reference notes  on forks

Updates `cross_reference_exists?` to match on commit only.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/5849

See merge request !2731

Ensure Commit#show responds 404 instead of 500 when given an invalid ID

Closes #13467

See merge request !2823

Closes #13469

Closes #13467