Add columns pages/artifacts size limits

What does this MR do?

Adds columns for per project/group pages size limits similar to repository_size_limit which we already have

#21688 (closed) #16610 (closed)

(moved from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16046)

Database checklist

• Conforms to the database guides

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body

== 20190911115056 AddProjectsMaxPagesSize: migrating ==========================
-- add_column(:projects, :max_pages_size, :bigint)
   -> 0.0012s
== 20190911115056 AddProjectsMaxPagesSize: migrated (0.0012s) =================

== 20190911115109 AddNamespacesMaxPagesSize: migrating ========================
-- add_column(:namespaces, :max_pages_size, :bigint)
   -> 0.0008s
== 20190911115109 AddNamespacesMaxPagesSize: migrated (0.0008s) ===============

== 20190911115207 AddProjectsMaxArtifactsSize: migrating ======================
-- add_column(:projects, :max_artifacts_size, :bigint)
   -> 0.0006s
== 20190911115207 AddProjectsMaxArtifactsSize: migrated (0.0007s) =============

== 20190911115222 AddNamespacesMaxArtifactsSize: migrating ====================
-- add_column(:namespaces, :max_artifacts_size, :bigint)
   -> 0.0005s
== 20190911115222 AddNamespacesMaxArtifactsSize: migrated (0.0005s) ===========

• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)