Add package file size limits settings to the admin UI

Plan limits for

maven_max_file_size
npm_max_file_size
nuget_max_file_size
conan_max_file_size
pypi_max_file_size

were recently added. Currently they are only able to be updated via the rails console or directly in the database via SQL. The admin UI should have these settings added for easier adjustment and higher visibility.

The most likely place to store these settings are in the admin package settings: https://docs.gitlab.com/ee/user/admin_area/settings/continuous_integration.html#package-registry-configuration-premium-only