We don't need a series of attributes to render diff files on
discussions.json request. Therefore this MR removes lots of unnecessary
attributes from the request, mainly the highlighted diff lines, which
are pretty expensive.

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.

This re-implements image commenting in merge request diffs.
This feature was previously lost when the merge request
page was refactored into Vue.

With this, we create an overlay component. The overlay
component handles displaying the comment badges
and the comment form badge.
Badges are displayed based on the position attribute
sent with the discussion.

Comment forms for diff files are controlled through
a different state property. This is so we don't
tie comment forms to diff files directly creating
deep nested state. Instead we create a flat array
which holds the file hash & the X & Y position of
the comment form.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48956

Display `formatter` as the sole content of `position` object.
This means `diff_file` data is not referenced, which is the
caseu of "IOError: not opened for reading".

This reverts commit 740ae2d1, reversing
changes made to 1ba47de5.