Backport changes in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2551 to CE

Closes #21391

See merge request !13275

Add Gitlab::Git::Blob.batch method

See merge request !13225

Improve mobile sidebar

Closes #35483

See merge request !13349

Resolve "Remove jQuery Nicescroll"

Closes #15664, #30860, and #35885

See merge request !13262

Upgrade to webpack v3.4.0

Closes #33902

See merge request !12298

Improve GitHub importer

Closes #27429

See merge request !12886

Allow projects to be started from a template

Closes #32420

See merge request !13108

Support group milestone references

Closes #34778

See merge request !13289

Set default options outside the raw_log method

See merge request !13346

Reset only migration models

Closes #36052

See merge request !13336

We had a lot of copied and pasted code, when the different elements were very
small and easy to get wrong.

Removes test for removed behavior

Add sidekiq metrics endpoint and add http server to sidekiq

See merge request !13082

Resolve "Navigation - Make contextual sidebar collapsible"

Closes #34028

See merge request !13281

add feature rename wiki title

Closes #27800

See merge request !10069

Bump gitlab-shell version to 5.8.0 to fix Git for Windows 2.14

See merge request !13344

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