Make dotenv variables and dotenv file size more easily configurable

Problem

We currently change the dotenv_variables and and dotenv_size via the Rails console:

• https://docs.gitlab.com/ee/administration/instance_limits.html#limit-dotenv-variables
• https://docs.gitlab.com/ee/administration/instance_limits.html#limit-dotenv-file-size

Admins, such as the GitLab Dedicated team, who exclusively manage GitLab instances through automated tooling do not have direct access to the rails console to make these changes.

Proposal

Make dotenv_variables and dotenv_size configurable via the UI in Admin Area.