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

Originally opened at !1677 (closed) by @kkm.

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As per #3071 (closed), 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 (closed).

mentioned in merge request !1677 (closed)

@dzaporozhets I reviewed, it works as advertised/expected. Please make final review/merge. Thanks!

Reassigned to @rymai

Added 2 commits:

• cf1349ad - Remember user's inline/tabular diff view preference in a cookie
• f6d816f9 - Add a controller test for the new 'diff_view' cookie

@dzaporozhets Changelog is good now! ;)

@rspeicher can you review please?

@rymai, @dzaporozhets: Thank you much guys for the help with the tests. I really know very little Ruby, and have near absolute zero experience writing tests for the Rails.

Added 2 commits:

• 24f048be - Update CHANGELOG
• 551ce0f2 - Add spec for assigning view param from cookie

Updated changelog entry and added a spec for assigning the view param from the cookie.

@rymai Rest LGTM, can merge when green.

mentioned in commit 9fdd605f

Status changed to merged

Thank you @kkm, @dzaporozhets and @rspeicher!