Refactor api events class to use external helper

Move specs from old class

Add changelog and magic string

Refactor events class to be more explicit

Remove blank line

We're filtering the events using `Event#visible_to_user?`.

At most we're loading 100 events at once.

Pagination is also dealt with in the finder, but the resulting array
is wrapped in a `Kaminari.paginate_array` so the API's pagination
helpers keep working. We're passing the total count into that
paginatable array, which would include confidential events. But we're
not disclosing anything.

It also renames the API::PROJECT_ENDPOINT_REQUIREMENTS constant to
API::NAMESPACE_OR_PROJECT_REQUIREMENTS

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.