Revert "Merge branch '44870-remove-extra-space-around-comment-form-on-merge-requests' into 'master'"

This reverts merge request !18120

This refactors the Markdown pipeline so it supports the rendering of
multiple documents that may belong to different projects. An example of
where this happens is when displaying the event feed of a group. In this
case we retrieve events for all projects in the group. Previously we
would group events per project and render these chunks separately, but
this would result in many SQL queries being executed. By extending the
Markdown pipeline to support this out of the box we can drastically
reduce the number of SQL queries.

To achieve this we introduce a new object to the pipeline:
Banzai::RenderContext. This object simply wraps two other objects: an
optional Project instance, and an optional User instance. On its own
this wouldn't be very helpful, but a RenderContext can also be used to
associate HTML documents with specific Project instances. This work is
done in Banzai::ObjectRenderer and allows us to reuse as many queries
(and results) as possible.

In Rails 5.0 the `ActiveRecord::Inheritance::subclass_from_attributes`
method was updated.
Now it calls the `find_sti_class` method [1] which is overriden in the `Event`
model and returns needed class (`Event` vs `PushEvent`).

[1]: https://github.com/rails/rails/blob/5-0-stable/activerecord/lib/active_record/inheritance.rb#L209

This commit fixes the errors like
```
143) User#contributed_projects doesn't include IDs for unrelated projects
      Failure/Error: action = attrs.with_indifferent_access[inheritance_column].to_i

      NoMethodError:
        undefined method `with_indifferent_access' for nil:NilClass
      # ./app/models/event.rb:118:in `subclass_from_attributes'
```
which are raised on the `RAILS5=1 rspec ...` command.

When a user is not logged in, we want to show the `New Issue` link so
he gets directed to the login flow first.

When a project is archived, we never want to show the link.

This prevents performing the requests, and disables all emoji reaction buttons

This could only be possible for users that can create merge requests
within a project.

So they need to be a allowed to create a branch and create a merge request.

When a project is archived, don't show the button on the index page
of merge requests

So we can distinguish between the permissions on the source and the
target project.

- `create_merge_request_from` indicates a user can create a merge
  request with the project as a source_project
- `create_merge_request_in` indicates a user can create a merge
  request with the project as a target_project