Correctly updates the URL without overwriting parameters
Reloads the diff file content without reloading the page

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58852, https://gitlab.com/gitlab-org/gitlab-ce/issues/42597

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58597

- Moved discussion notes out of `NoteableDiscussion` component into a
new `DiscussionNotes` component
- Wrote Jest tests for the new `DiscussionNotes` component
- Updated Jest config for emojis fixtures
- Updated Karma tests `NoteableDiscussion` to match its new structure
- Convert `DiffDiscussions` tests to use Vue test utils

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

- Compare versions header is full width except in the unified diff mode
with no tree sidebar
  - Bar is always full width, but the content within stays centered when
unified and no tree sidebar
- File header is the same height as the "Compare versions header"
- aligns with the design system grid guidelines => 56px
- Diff file headers use a button group, switch icon order to open file
externally being the last option, all buttons will become icon buttons
(icon delivery by @dimitrieh)
- If a file header becomes sticky no rounded corner/double border
problem is visible anymore

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59802

Fix bug where clicking file header in diff refreshes page

Closes #57669

See merge request gitlab-org/gitlab-ce!26422

- Add an ID to the diff content
- handle clicking on file name in diffFileHeader when it is not a link
to another page but rather a link to an element on the page