Refactor events to use enums

This ensures that the methods we use for retrieving events respect the
:wiki_events feature flag.

This flag must be applied in two places - both in the event_collection
and in the events_finder, since there exist places in the code base
where the event_collection is used without the finder (for example in
the projects or the groups controllers) so it must be applied there, but
in the events_finder there is a branch that by-passes the
event_collection, so we also need it in the finder.

To support this flag, we have a new scope `Event.not_wiki_page`, which
only returns events that are not wiki page events.

This flag is off by default.

This includes improvements to the event collection spec by asserting
identity rather than length of results.

Includes suggested changes from reviewer (@.luke)

Co-authored-by: @.luke

The API returns a 500 error when scope is supplied and current_user
is not present. This is because a user's projects are used in the
query that returns all events.

This change adds a check to confirm that current_user is provided
before trying to run the query when scope is passed

Currently, `/events`returns the activity of an authorized user.

This changes the endpoint to return activity similar to a user's
dashboard on the UI. It returns the activity across the
authenticated user's accessible projects

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.

Partially addresses #47424.

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

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

This ability can be disabled when an external authorization service is
enabled.

Introduced in
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2201

This optimises how GroupProjectsFinder builds it collection, producing
simpler and faster queries in the process. It also cleans up the code a
bit to make it easier to understand.

This optimises how GroupProjectsFinder builds it collection, producing
simpler and faster queries in the process. It also cleans up the code a
bit to make it easier to understand.

* Meld the following disparate endpoints:
 * `/projects/:id/events`
 * `/events`
 * `/users/:id/events`
+ Add result filtering to the above endpoints:
 * action
 * target_type
 * before and after dates