Merge branch 'kkm/gitlab-ce-user-diff-view-pref-cookie' into 'master'

Remember user's inline/tabular diff view preference in a cookie

_Originally opened at !1677 by @kkm._

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As per #3071, some users (we have a local EE installation) prefer 2-column view in diff. In this MR I add an implementation for this feature, using a cookie.

Fixes #3071.

See merge request !2723