Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56523

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52916

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53304

Dropdown includes buttons for tree view rendering mode
and buttons for the compare view (inline or side-by-side)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55491

The empty state now only gets shown when no files exist in the branch.
If the user is reviewing 2 versions with no files, we don't show the state.

Refactors the diff app spec to use Vue test utils.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48635

This caused many pain points when working with it.
Part of the data was camel cased the other snake case.
Other parts where snake case & then getting converted in components,
this conversion has the potential for leaking memory.

This changes that & makes it consistent with what it returned from the
API, snake case.