Introduce user preference "collapse_diff_by_default"

Joe Snyderrequested to merge
gitlab-community/gitlab-org/gitlab:add_collapse_diff_by_default_user_setting_merge_requests into master

What does this MR do and why?

Introduce user preference "collapse_diff_by_default"

Introduce a new User preference that would automatically collapse diff entries on load of a Merge Request's changes.

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