Add diff hunks to notification emails

Add diff hunks to notification emails. Continued from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5855 - thanks @hoopes!

This also fixes an issue where the + / - prefixes were missing from diffs in emails.

Screenshots (from my browser) of the HTML emails, along with text screenshots 😛

![image](/uploads/cb31400becf5149d40c8bb98a655aa93/image.png)

```
New comment for Merge Request !1 on app/views/admin/builds/index.html.haml:

http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/1#note_1023

>          Finished

This is a comment at the top of a match section.
```

![image](/uploads/704dd3845797530697a27f5c1953c053/image.png)

```
New comment for Merge Request !1 on app/views/admin/builds/index.html.haml:

http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/1#note_1022

>          Finished
>          %span.badge.js-running-count= @all_builds.finished.count(:id)
>  
> -    %li{class: ('active' if @scope == 'all')}
> -      = link_to admin_builds_path(scope: :all) do
> -        All
> -        %span.badge.js-totalbuilds-count= @all_builds.count(:id)
> -
>  .gray-content-block
>    #{(@scope || 'running').capitalize} builds
>  

This is a comment at the bottom of a match section.
```

![image](/uploads/4063f3d9738aea8ebf3c0e690d0eddee/image.png)

```
New comment for Merge Request !1 on app/views/admin/builds/index.html.haml:

http://localhost:3000/gitlab-org/gitlab-ce/merge_requests/1#note_1024

>          = link_to 'Cancel all', cancel_all_admin_builds_path, data: { confirm: 'Are you sure?' }, class: 'btn btn-danger', method: :post
>  
>    %ul.center-top-menu
> -    %li{class: ('active' if @scope.nil?)}
> +    %li{class: ('active' if @scope == 'all')}
>        = link_to admin_builds_path do
> +        All

This is a comment with some deleted and added lines above it.
```

Closes #21027, closes #24340.

See merge request !7660