[Rails5] Add `safe_params` helper

See merge request gitlab-org/gitlab-ce!18241

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.

Rails 5.0 requires to explicitly permit attributes when building a URL
using current `params` object.

The `safe_params` helper allows developers to just call `safe_params.merge(...)`
instead of manually adding `permit` to every call.

https://github.com/rails/rails/pull/20868

Previously, these wouldn't count issues or MRs in subgroups - meaning that if
_this_ group had no issues or MRs, we'd show the empty state, which was wrong.

The group activity feed was limited to the first 20 projects found
in the group, which caused activity from some projects to be omitted.
A limit of 20 is applied to the query for events, so the
extra pagination does little in the way of performance.

Closes #42560