Add columns for pages size limit

What does this MR do?

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

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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

== 20190903095845 AddProjectPagesSizeLimit: migrating =========================
-- add_column(:projects, :pages_size_limit, :bigint)
   -> 0.0075s
== 20190903095845 AddProjectPagesSizeLimit: migrated (0.0076s) ================

== 20190903100026 AddNamespacesPagesSizeLimit: migrating ======================
-- add_column(:namespaces, :pages_size_limit, :bigint)
   -> 0.0015s
== 20190903100026 AddNamespacesPagesSizeLimit: migrated (0.0016s) =============

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