Closes #41561

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This makes navigating through diff files quickly. Currently we just
toggle a list, which could be pretty big. This moves it into a dropdown
to make it much easier.

Also includes a filter bar to quickly search for certain
files/extensions.

Closes #29778

When you change the diff view (inline / side-by-side), we set a cookie based on
that new view. When you add a comment, we choose the style to use in the
response based on that cookie.

However, when you have just changed diff style, the request cookie will contain
the old value, so we should use the view param instead.

Use the EnforcedStyleForMultiline: no_comma option.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Don't accidentally mark unsafe diff lines as HTML safe

Fixes potential XSS issue when a legacy diff note is created on a merge
request whose diff contained HTML

See https://gitlab.com/gitlab-org/gitlab-ce/issues/25249

See merge request !2040

In the browser, we remove the + and - signs from the front of a diff
line because we add them in with CSS, so they aren't copied. We can't do
that in an email, because the CSS isn't supported, so we should keep
them in that case.

So we have raw_diffs too

Instead calling diff_collection.count use diff_collection.size which is cache on the diff_collection

Introducing the concept of SafeDiffs which relates 
diffs with UI highlighting.

1. Move render method to a concern, not a helper.
2. Let DiffHelper#diff_options automatically add the path option.
3. Move more instance var definitions to before filters.

Other diffs (those that are too large to render anyway, image diffs,
diffs suppressed by .gitattributes) should be rendered immediately.