== Public Projects

This finder class now _only_ returns public projects. Previously this
finder would also return private and internal projects. Including these
projects makes caching data much harder and less efficient. Meanwhile
including this data isn't very useful as very few users would be
interested in seeing projects they have access to as trending. That is,
the feature is more useful when you want to see what _other_ popular
projects there are.

== Caching

The data returned by TrendingProjectsFinder is now cached for a day
based on the number of months the data should be restricted to. The
cache is not flushed explicitly, instead it's rebuilt whenever it
expires.

== Timings

To measure the impact I changed the finder code to use the last 24
months instead of the last month. I then executed and measured 10
requests to the explore page. On the current "master" branch (commit
88fa5916ffa0aea491cd339272ed7437c3f52dc7) this would take an average of
2.43 seconds. Using the changes of this commit this was reduced to
around 1.7 seconds.

Fixes gitlab-org/gitlab-ce#22164

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>

Refactor ability.rb into Policies

## What does this MR do?
Factors out `ability.rb` into a new abstraction - the "policy" (stored in `app/policies`). A policy is a class named `#{class_name}Policy` (looked up automatically as needed) that implements `rules` as follows:

``` ruby
class ThingPolicy < BasePolicy
  def rules
    @user # this is a user to determine abilities for, optionally nil in the anonymous case
    @subject # this is the subject of the ability, guaranteed to be an instance of `Thing`
    can! :some_ability # grant the :some_ability permission
    cannot! :some_ability # ensure that :some_ability is not allowed. this overrides any `can!` that is called before or after
    delegate! @subject.other_thing # merge the abilities (can!) and prohibitions (cannot!) from `@subject.other_thing`
    can? :some_ability # test whether, so far, :some_ability is allowed
  end

  def anonymous_rules
    # optional. if not implemented `rules` is called where `@user` is nil. otherwise this method is called when `@user` is nil.
  end
end
```

See merge request !5796