When displaying a Group details page we check if epics are enabled for
the group and all of its descendants. For this set we will eventually
run at least one SQL query per Group to get the plans said Group has
access to. In case of a hierarchy with 20 groups this means at least 20
SQL queries.

In this commit we introduce Gitlab::GroupPlansPreloader. This is a
custom class that takes an ActiveRecord::Relation of Group objects and
efficiently preloads the plans for these groups. This class only needs
three SQL queries to do its work:

1. A SQL query to get the groups and all of their ancestors.
2. A SQL query to get all the plans all of these groups have access to.
3. A SQL query to get the groups to assign the plans to, then return to
   the caller.

Using this data the class can then determine which groups have access to
what plans.

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/6206